The only time it happened to me was a slow leak. I just kept dumping the excess air until the dive was over and then replaced the inflator with a spare. It did take a little while to figure out what was going on, though.
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Going slightly head down also allows you to swim down while finding the dump and closing primary valve, as well as facilitating dumping.Don't go vertical while doing this. Stay in trim as that will slow down an uncontrolled ascent. (Maybe go slightly head down if that makes reaching your valves easier.)