A course of action that would appear to lead to a good outcome, but when undertaken, leads to a situation that is utterly out of control beyond human comprehension.

A gigantic cluster fuck... but not in a military sense.

A huge fuck-up of epic proportions of some sort or another and its ensuing calamity. As in, the person or normal situation you were supposed to be a part of is now so totally screwed up as to turn the entire scenario into a farce; or something that could end up in you going to either jail or losing your profession.

A shitstorm refers not to one bad thing happening, but to many bad things happening in a short period of time. Usually describes many unlucky, sad or just awful events pertinent to the person speaking.

A situation where oneself or something of value to self becomes attacked by a large quantity of basically crap, which must be fended off at a high rate in order to return the system to normal.

When a tornado hits an exposed septic tank and sends all the contents into the air, where it falls down in a rain-like pattern.

A discussion about something totally pointless and inane that got out of hand and now consists of people fighting over nothing.
Basically like a flamewar, just that the discussion itself, as well as the participants, are so utterly stupid that it's impossible to put into words. Shitstorms like these always tend to be really easy to start, even when you don't want it - just like an avalanche, one false step (or word) can be enough to set free a shitstorm of epic proportions. One easy example for this is the ongoing console-fanyboy war.

When something completely unforeseen goes horribly wrong beyond mortal comprehension in something that should be normal and routine. This is the ultimate form of 'Murphy's Law".