This technically isn't what I would call a bug, more of an accidental happening...
Here it is:
When using a custom window with the debug command, ie, /debug @whois - The debug output, as well as any output generated by script that is directed to a window labelled @whois, will appear on the same window.
Call it a bug, call it a feature, or call it plain annoying, do as you please, but I fall into the 'slightly annoyed' category.
Perhaps the /debug command should be given its own windowing subsystem, and that any window opened up by /debug cannot have 'accidental output' generated to it by any script within mIRC.
