I actually did use the fact that MacVim is open source to fix a bug one day. Well, actually, someone else had already figured it out and provided a patch, but

I actually did use the fact that MacVim is open source to fix a bug one day. Well, actually, someone else had already figured it out and provided a patch, but it wasn't in the main line yet and I really, really needed it that day. But I was already on the track to fixing it when I found someone else's patch.