My worst time DMing has to be, hands down, the first time I ever tried a war campaign.

The PCs were supposed to be functioning as an elite commando unit. What'd they do? They went straight to the front lines, created a gigantic mud pit with magic, and proceeded to try and hold off the orc horde.

We were swamped in rolls. Couldn't get a damned thing done. Campaign didn't go for a second session. And I'd put so much work into it beforehand, too. The surrounding terrain, multiple copies of maps of said terrain, the orc warlord's bodyguard, the strategy of the grossly outnumbered town militia to evacuate the town and pull back across the river, the whole lot. All to waste because most of the party just decided to charge the nearest orc.