It only takes one time. The first fixed wheel I had was one of the entry level flip flops from Suzue (yea, Suzue - Suzie's a girl)

After a couple of months on it I spun it off just backpedaling on a downhill. Luckily there was a LBS a few blocks away that had a guy that rode fix, and he was able to kind of get the cog and lockring back on there, the threads weren't completely gone, but it wasn't safe either. Luckily, that was back when I still rode with a front brake - I rode it for a couple of weeks while I saved my pennies for a Surly. I think it's still in Goatmeals basement actually, waiting for the rim to get laced to a working hub...

I dunno - I just don't trust anything entry level from Suzue. When my new Surly dissappeared when my bike was stolen, I bought a wheel off of e-bay with a Suzue because money was tight. I only rode it for about 6 months, but it is pretty much toast (to be fair, it did start life in a Minnesota fall, then live through the winter) and I'm just going to use it as a winter beater wheel. The bearings are shot on this one, not the threads, but still...