I was writing from experience! Emptying a supposedly nearly empty tank into a 25L container. Plenty of room, no worries ... oh bugger!
Yebbut not before he's got wet shoes that stink of petrol. It's uncanny how petrol that's taking ages to syphon suddenly speeds up the moment you need to stop the flow.
Or in my case realising that the 25L plastic container now weighed 25Kg and I'd need to lift it high enough to syphon back into the other car.