You finish last week's setup, open your first Blueprint, wire up some nodes — and the editor lights up red. Welcome to the second wall every UE beginner hits. Most of these errors look terrifying and are actually simple once you know what they're saying. Here are the ten you'll hit most.

1. Accessed None — "tried to use an object that doesn't exist"

The single most common Blueprint error. Format: Accessed None trying to read property X from Y.

It means: you called something on a variable that was null. The variable was never set, or whatever you cast from returned None.

Fix: Right-click the offending pin → Promote to local variable → before using it, branch on Is Valid. The cleaner pattern: drag off the variable → search "is valid" → drop the ?Is Valid node. It has a green check pin (run if valid) and red X (run if not). Wire your logic into the green.

Don't suppress it with Is Valid blindly though — first ask why it's None. 80% of the time you're trying to use the variable before something else set it.

2. Cast Failed — "this object isn't what you thought it was"

Every Cast To BP_X node has a Cast Failed execution pin. If your code lands there, the input wasn't actually of type BP_X.

Fix: Most Cast Failed errors aren't errors — they're informational. The cast tried; it didn't match; the code took the failure branch. That's fine. If it surprises you, the real problem is that you assumed every overlap / hit / interaction would be with type X, and it wasn't.

Anti-pattern: chaining Cast To to absolutely everything to access functions. That creates hard references and slow loads. We'll cover the alternative — Blueprint Interfaces — in a future post.

3. Infinite Loop Detected

Infinite Loop detected — script has hit a limit of 1000000 iterations. Unreal halts your script before it hangs the editor.

Three causes, in order of likelihood:

  • A loop with no exit condition. Check your While loops. Forgot to increment the counter?
  • A function calling itself directly or indirectly. OnSomeEvent triggers UpdateUI which triggers OnSomeEvent again.
  • An event firing in Tick. You broadcast an event from a Tick — something listens — and that listener does something that broadcasts again from the same Tick. Loop.

Fix: Add a counter and break out after N iterations while debugging. Print the loop body's state. Find the cycle, cut it.

4. TRASHCLASS / Compile errors after renaming

You see TRASHCLASS_BP_X_0 in a node title, or the Blueprint won't compile with "class not found".

It means: you renamed or moved a Blueprint and a reference somewhere still points at the old path. Unreal turned the now-orphaned reference into a "trash class".

Fix: Reload the parent project (close + reopen the editor). If still broken, open the broken Blueprint, find the trash-class node, right-click → Refresh Node, or just delete and re-add the reference. Save everything. The trash usually disappears on the next save.

5. Variable is not set / "Default Value" warning

A variable shows a yellow triangle in the Details panel. The compiler is warning you that something references a variable that has no default and isn't set anywhere before use.

Fix: Either set a Default Value in the variable's details, or guarantee it's set in BeginPlay or the Construction Script before any other graph reads it.

6. Construction Script crash on PIE start

You hit Play and the editor freezes for a moment, then crashes — or the actor doesn't spawn correctly. Often the cause is heavy work in the Construction Script: spawning actors, accessing world state, calling Tick-only nodes.

The Construction Script runs in the editor, every time you move the actor in the viewport. It runs before the world is fully alive. So Get Player Character returns None. So Spawn Actor may misbehave.

Fix: If the work is "world-aware" or runtime-only, do it in BeginPlay, not Construction Script. Reserve Construction Script for editor-time setup (e.g. updating preview meshes when properties change).

7. "Could not load asset" on package

Editor runs fine. You package the game. The packaged build doesn't load some asset. The log says Could not find asset Game/...

Two common causes:

  • Soft reference to an asset in an unloaded chunk. Either make it hard, or ensure the asset is in the cooked chunk list (Project Settings → Packaging → List of maps to include).
  • Folder named with a leading underscore (_Developers/). These folders are excluded from cook by default. Don't ship anything from inside _Developers/.

Fix: Check the log for the full asset path. Either move the asset out of an excluded folder or add it to the cook list explicitly.

8. Pin connection failure — "incompatible types"

You drag from an output pin to an input pin. The wire flashes red and refuses to connect. Status bar: X and Y are not compatible.

The two pins are typed and don't auto-convert.

Fix options:

  • Right-click the destination pin → Convert from X if Unreal knows a converter (Float to Int, Vector to Rotator, Object to specific class via Cast).
  • For Object → specific class: drop a Cast To node in between.
  • For Int → Float and similar: just drop a conversion node.

If you see this between BP nodes that "should" connect, you probably have an array vs single-element mismatch (the most common gotcha — array Items of type Item won't directly connect to a pin expecting Item).

9. Animation Blueprint not updating

The character's animation doesn't change at runtime, even though you set the variable. Or it changes once then sticks.

Animation Blueprints have a special graph called the AnimGraph, which is sampled every frame, and an Event Graph for setting variables. Beginners often set the variable correctly but never wire it into the AnimGraph state machine transitions.

Fix: Open the Animation Blueprint. Switch to the AnimGraph. Confirm the state machine actually checks your variable in a Transition Rule. Print the variable in the Event Graph to confirm it's being updated.

10. "Reference would create a circular dependency"

You try to reference Blueprint A from Blueprint B, and B already references A somewhere. Unreal blocks the new reference because resolving them would loop forever.

Fix: Break the cycle by introducing an intermediate. Common patterns:

  • An Interface that both A and B implement — they talk through the interface without referencing each other.
  • A Subsystem (Game Instance Subsystem) that both query — neither references the other directly.
  • An Event Dispatcher on A that B binds to — B knows about A but A doesn't know B.

If you can't break the cycle, you have an architectural problem worth thinking about — not just a Blueprint quirk.

Common pitfalls

  • Suppressing errors with Is Valid everywhere. Sometimes the right answer is to find out why it's None, not to silently skip the logic.
  • Print String for everything. Useful for one-off debugging but leave them in shipping and you've created a log spam. Use Print String with the Print to Log and Print to Screen toggles consciously, and delete them when done.
  • Treating compile warnings as background noise. Compile is fast. Warnings free. Fix them; they often catch real bugs before they're bugs.
  • Editing assets without saving the level too. Many "my changes disappeared" bugs come from saving the Blueprint but not the level that placed it. Use Save All.

The 30-second recap

  • Accessed None → variable wasn't set. Branch on Is Valid and fix the root cause.
  • Cast Failed → not an error; just a no-match. Don't chain casts.
  • Infinite Loop → exit condition missing, or event cycling.
  • TRASHCLASS → stale reference; refresh node or restart editor.
  • Construction Script crash → move runtime work to BeginPlay.
  • Cook missing asset → folder excluded or soft ref not in chunk list.
  • Pin types incompatible → use a conversion node or Cast To.
  • AnimBP not updating → wire variable into AnimGraph transitions, not just Event Graph.
  • Circular dependency → break with Interface, Subsystem, or Event Dispatcher.

These ten cover ~90% of what new Blueprint devs hit. Memorise the pattern, not the error code.

Next week: Enhanced Input System — the new way, simplified. Once you can wire up logic without errors, the next thing you want is to actually control the character. Spoiler: the new input system replaces the old one for good reasons, and most beginners get confused by the four-concept stack (Action / Context / Trigger / Modifier). Next Sunday it'll click.

— Marco