Well, for example Meld crashes when you run a comparison if you use any of a half-dozen classic styles that often still ship with distributions, due to changes in the underlying GTK libraries. And that's just ridiculous; a GUI toolkit that is so incapable of handling older then-valid styles that it crashes applications? That's not a very confidence-inducing GUI toolkit.
That's the only really recent one I can think of, but that's largely because the issues I've run into peaked about 2 years ago, at which point I largely culled GTK applications from my list of applications that I commonly use (GParted persisted for a while longer, but these days the KDE Partition Manager is on par, in fact I believe it has slightly surpassed GParted although admittedly not for anything I actually use so I can't remember the details). I've forgotten a lot of the issues since then, although they did largely come down to crashes and interface layout and iconography glitches.