Infected?

I had a blonde ale that had a very high residual sweetness and finishing gravity ( Shooting for FG of 1.012 and ended at 1.021 instead).

Three weeks after sitting in the keg (that i had switched out all o-rings, pbw'd, and starsan'd before filling), it has some wild off flavors. It is BAD.

Any ideas of what could be going on?

The only thing I can think of is that I use flare fittings on my tubing and that some wild yeast got in the tubing when a connector wasn't attached and then infected my entire keg. Thoughts?

When you say "bad", how does it taste?

You could get an infection at any point in the process (after the boil), so it's anyone's guess. About the only thing you can do is (next time) go through everything and make sure it gets sanitized.