I've never had an infection that I'm aware of.
I've had off flavors, and I've wondered, but nothing confirmed.
Most recently, I made a porter that gives off a brief sour twang to the aroma, but the taste is ok. I just kegged it today, so we'll see how it fares. I'm thinking that was due to a third generation yeast strain and a funny fermentation, rather than a true infection.
Funny, I had the same off aromas in my doppelbock-- its still lagering, and the aroma is fading away.