That happened to me, once. The weights slid off of one side just fine, then the imbalance causes the bar to literally jump up and flip off of my chest, it was kinda funny, although the gym guys were not too happy about it. Oh, and I have also used a power rack/cage. But sometimes, its just not available. Spotters are nice too, but again, sometimes I gotta go it alone.
Everything in moderation.
Notice how fast gravity effected the weighted end? Sure it's noisy and people give you strange looks but it is the safe way to go. It's better to say "excuse me" than for someone to say about you "he's dead".
Keep on lifting.