I use 'q' about once a week now, although that use to happen to me all the time too!

the way it works:
. you hit 'q', and then another character, like 'a'
. you'll see 'recording' at the bottom of the window, and vi is recording your keystrokes
. when you're done with what you want to record, hit 'q' while in command mode again.
. after that, you can get vi to do the same command again by using '@' and then the character you picked before.

I mostly use it when I want to change something on every line, but don't have the regular expression fu.