The Flip Side
I had this happen once as well, and I had the Flip Side of this situation occur, the DM's nightmare. I had a group that always planned things, was very cautious and used the best of their resources, and always made sure that they had the right spells prepared, equipment, that everyone was healed, if they needed to rest, etc. that they did it before they tackled anything major.
Then came the night. The one night that they decided to listen to the one unstable character in the group who was always advocating the 'seize the day' philosophy. I was running Against the Giants, the Liberation of Geoff, and they found the city run by the Fire Giants (I forget the name). Well they go in, discover the agents of the two covert groups that are in the town, they meet the priest of pelor who is pretending to not be a priest, and this is after several days travelling. The one character, a Barbarian, recommends attacking the Fire Giant fort. The spellcasters do not have battle spells ready. A few characters are low on hit points. This will be the culmination of about 9 months of real life game time, a major point in the campaign.
I figure they'll dismiss the barbarian, preferring to memorize the appropriate spells, heal up, recoinnoiter, etc. since the Fire Giant King and his elite guards are in the castle. Fully buffed and prepared, they could handle it. It might be tough, but they'll win.
Nope. They listen to the barbarian, blow the cover of the secret agents and charge the castle.
They got wiped out.
And blamed me, at least a couple of them did.
I tried to place subtle warnings, have the townspeople tell the horrors of the giant's behavior, the hidden agents explained the terrors of the occupation, etc.
They ignored it all anyway, for the only time this group has ever done something like that.