I just whined about this elsewhere, but this makes a good post, so:
1. a very good feature of default Win32 message boxes is that they implement Ctrl-C to copy the error message and window title into the clipboard (formatted and everything)
2. way too few people know this, and accordingly, custom message boxes basically never implement this
3. if you work on custom UI libraries that have some message box facility, you should!

@rygorous I am in fact pleasantly surprised how many games that have copyable (yes I can verbify anything!) error codes in their (custom) message boxes. You kinda assume they aren't.

Too few, but still more than expected.