Since I'm still starting out with this hobby there have already been a few. I do read reviews and try to buy the best gear at a reasonable cost, but the issue comes up when I start using the items. Don't know what I want and what not.
Bought some zip-off trousers for warmer weather - utter crap, nettles sting through them. Never buying pure synthetic again. (Even though they seemed great at first - quite cool and dry quick, I sweat like a pig.)
Similar problem with a waterproof jacket I got, I just get too hot in there. Definitely not buying any more membrane jackets. (Still good to have though, when it starts pouring down, but for overall use it's nearly worthless.)
The rest of it has gone smoothly and I've gotten much more gear that I'm thoroughly satisfied with than stuff that is just bad.