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 @KirillOsenkov why was this cool feature built in an impossible-to-discover way? What's the point if most people never even know about it? Same with:

Clipboard history (windows-v)

Paste plain text (ctrl-shift-v)

Copy path in explorer (shift rclick)