Luau traps that broke real games

Language

• A local used before its declaration is a nil global. One function referencing a helper declared further down the file read nil and threw thousands of frame-loop errors. Declare locals above their first use (or forward-declare them).
• (x :: T).Prop = v on the line after one ending in ) parses as a call. Luau reads it as calling the previous expression. Assign the cast to a local first.
• Weak-keyed tables for registries that must persist end up empty. The garbage collector clears them. Use strong tables and prune entries when Parent becomes nil.

Engine

• PreloadAsync fetches but doesn't decode. Draw each unique image once in a hidden ScreenGui at boot to really warm it up.
• Unused server-to-client remotes need a no-op listener. Otherwise the client logs "Remote event invocation queue exhausted" and drops events.
• Know whether an event carries the before or the after value. A server that patches state before firing sends the new value; UI that assumed the old one showed wrong numbers.
• Let replicated state land before reading it. A short task.wait(0.15) before a popup reads counters avoids showing the previous value.
• Some phones can't run --!native. Leave it off small client modules.

Defensive habits

• Rate-limit warnings in frame loops (once per message) and wrap RenderStepped bodies in pcall. One error at 60 fps buries everything else in the output.
• Pooled objects need a generation counter so delayed callbacks from a previous use don't act on the object's next life.
• Guard optional APIs (if UI.PageChanged then). An older copy of a module without the function killed a whole bridge.
• Find UI by exported name, never by GetChildren order or sorted index.
• Tool output created in edit mode gets Archivable = false so harness runs don't leave debris saved into the place.