Summary
Bug Type: Fatal Out-of-Bounds Assertion Error (Memory/Data Structure Mismatch).
Component: MeshModelingToolsEditorOnly (Skeletal Mesh Editing Tools Plugin).
The Cause: The editor internally miscalculated vertex or bone indices during a weight assignment operation. It attempted to read data index 271 inside an internal memory list that capped out at index 270.
The Result: To prevent project data corruption, Unreal Engine’s safety manager triggered an intentional assertion failure, immediately shutting down the editor.
What Type of Bug are you experiencing?
Level Design & Art Tools
Steps to Reproduce
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Open a Skeletal Mesh asset inside the editor with the “Skeletal Mesh Editing Tools” plugin enabled.
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Open the “Editing Tools” workspace and select the “Skin / Edit Weights” tool.
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Select a set of vertices on a combined mesh geometry.
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Select a target bone from the bone list and click the “Add” button to assign a numerical weight value.
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Click on Accept.
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The editor crashes instantly with an out-of-bounds array assertion error.
Expected Result
The tool should assign the weight value of 1.0 to the selected vertices without crashing
Observed Result
The engine instantly throws a fatal assertion crash: Array index out of bounds: 271 into an array of size 271 inside MeshModelingToolsEditorOnly.
Affects Versions
5.6
Platform(s)
Windows
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NiceAllDay.log (120 KB)
For crash reports, include your callstack
Assertion failed: (Index >= 0) & (Index < ArrayNum) [File:D:\build++UE5\Sync\Engine\Source\Runtime\Core\Public\Containers\Array.h] [Line: 1067]
Array index out of bounds: 271 into an array of size 271
UnrealEditor_Core
UnrealEditor_MeshModelingToolsEditorOnly
UnrealEditor_MeshModelingTools
UnrealEditor_InteractiveToolsFramework
UnrealEditor_UnrealEd
Additional Notes
CPU: AMD Ryzen 7 7840HS
OS: Windows 11
Project Build Configuration: Development