Wow, that's interesting to hear. I have a 50 1.4 where the AF got flaky. It started when I took it to the beach and it got a few grains of sand in the lens from a blowing wind.
Strike #1, no weather sealing, very easy to get dust/dirt inside
Eventually I cleared the sand, but holding it face down and shaking and what not. Didn't see the sand exit the body, must have been a few grains. Until then the lens was locked up, the USM wouldn't work. Anyhow with the sand cleared out, the AF started to work again, intermittently.
Strike #2, AF fails after about five minutes of use, every time.
The lens kicks around, I've thought of sending it in, but it wasn't expensive in the first place and didn't seem worth it. Now I have the 24-105L and get much better pictures, colors in particular are better on this lens, plus the weather sealing.
The only thing the 50 has going for it is that it is small. It's fast, but you don't want to use it wide open as DOF is too narrow.