it happened to me once, i was clearing my rena filter, and i was running co2 reactor, well the rena was empty of water, even the inlet pipe to the filter was empty, i've tried to fill the hose and fill the small cap of the pipe, but it didn't work.. because the CO2 Reactor was full of water, and the air didn't get out...
So I tried to s.ck the hose, and YAACK! what a taste of planted tank!