Hmm. the earliest i remember wishing i knew that before... Not that i remember it all too clearly, but here it is:
back in the 2D version i think it was, i had a system of moats set up around a castle that i'd built, that had secondary channels set up to act as traps/means to keep my dwarves indoors, and being in a hurry to finish one as i was worried that i'd get a seige, i'd installed a door instead of a floodgate in one of the spots. i had double bridges raised on either side, to stop them being destroyed because i think this was my... magma moat.
anyway, this particular 'moat' was set so water'd keep flowing into it without solidifying it as it was in a channel (as i recall, overflowing water wouldn't set magma in a channel solid in the 2D version, due to the way channel's worked.) once i set it off, thus causing scalding clouds of steam to boil anything that tried to head down that route, as a way of doubly protecting it as well as giving me a frontal steam protected entrance.
I found out after the first run, that the door, even when closed and locked wasn't stopping the flow of fluid. This wouldn't have been a problem, if it wasn't for the fact that the first run, was caused by something destroying one of my floodgates. Toggling the bridge/door seemed to do nothing, and my last resort attempt to cave in the area ALMOST worked. apart from the fact that the water was coming on the inside of the magma channel, and thus directly into my fort.
I've been wary of undertaking fluid based projects ever since.