Out of about 30 batches brewed so far, one had the classic signs of infection. Another had the 'band aid burps' that I think was infection, but may also have been cleaner or sanitizer contamination. The definitely infected batch had the telltale ring at the bottle neck and sour taste.
Possible off flavors from small scale infections bug me the most. These don't ruin your beer outright, but can skew flavors; especially as the beer ages.
Just heard a podcast by Jamil Z. in which he says he is so suspicious of junk lingering in bottles that he uses new bottles for every brew! His beers are apparently known for aging well over unusually long time periods; whether in bottle or in kegs.
My airlock passes gas.