I have never (yet) actually puked from a workout. I have come close. I hope to keep it that way.
I would guess that about once every month or two, I end up pushing hard enough that it becomes a distince possibility that I'm going to heave. I usually back off a bit at that point . . . I hate throwing up. Sometimes I know I'm going to push to that edge, sometimes the edge turns out to be closer than I thought.
Like Jeremy, ideally, I think you need to push it really hard at least once or twice a month. Pushing hard on a "metabolic burnout" kind of workout will probably produce a "near occasion of Pukie" (to paraphrase my religous training). It doesn't happen for me with a max effort on a "focus day" . . . only the "row 500 m/200 thrusters/50 burpees, repeat 10 times" kind of WODs.