I've heard a few others mention this, but had never had it happen. But I had my first overnight vivi leak last night. Leaked most of a tank out. I cleaned it, put on another head (not a new one) and it's working fine, no leaking, so don't know what caused it. I had just filled it not long before, so perhaps something wasn't screwed on just right, but it didn't leak while I was using it, just as it sat sort of at an angle overnight. Luckily I always remove juice holders from my pv overnight, so it leaked into a case. From now on will set it in a shot glass just to be safe.