Macs have become a lot worse at this sort of thing - weird, intermittent user interface failures - over the last few years.
Sometimes I can't drag attachments out of emails (which is about the only damn reason I don't use Mutt for everything) on the first few attempts; I thought for a while it was because the attachment hadn't completely downloaded, but no, it doesn't seem to be as predictable as that.
Moving files around in the Finder in general seems problematic, especially dragging between directories in the list view. This often causes much cursing.
The worst thing about Macs for me now is how much guff pops up entirely unbidden - messages about not having done any backups in 90 days, messages about an update which hasn't been able to be completed (but has caused iTerm to try and quit, which, in itself, causes a dialog to appear that I need to get rid of), iCloud approval nonsense (which also happens on both of my iPhones periodically), iTunes Store requests that I may or may not have made days ago that have failed with an unknown error, etc. The list goes on. I'm sure all of these things in isolation are very clever and should help improve my life, but in toto it's immensely frustrating.