Happened some time ago. Did the weekly WC no problems. Doing the WC the week after that, can't get a siphon out of the tank, tried everything. Smell something funky, so I disassembled the valve on the hose, turns out one of my golden labs got sucked up the week before and died. I made a ghetto safety net by cutting up a net before I started the WC.
Not as serious. Did a WC, disconnected the hose from the tap and walked away, turns out this created a siphon and drained all the water I just put into the tank. I had to refill it again the same day.