Pretty much what it says.
For me, it was Spelunky 2. Spelunky HD is arguably one of the best games of its generation, and 2 was hyped up beyond belief for what felt like half a decade. Then it came out and it was just crap. Spelunky was hard, but felt fair. 2 was just rubbish. It felt like you were just being killed with no rhyme or reason.
I actually think Spelunky 2 is the most disappointing sequel of all time