Critical Failures: What Color Was That Dragon, Again?

This happened like, the session before we stopped running my first homebrew 4th Edition campaign over a year ago. I had gotten Player's Handbook 2 months before and was itching to run something with a tight primal/Feywild/celtic theme. The main quest was that the players were searching for pieces of a song that had been written by a long-dead bard that for some reason a bunch of other major players--eladrin, drow, and fomorians to name a few--were also looking for.

Over the course of some ten levels, they plundered an eladrin crypt, found the ashes of someone who knew the location of one part, had a ghostspeaker pry the information out of it, liberated a town from a haunted castle, navigated a forest half stuck in the Feywild, delved into an ancient druid pyramid, and eventually found the first piece. They also found a map with two parts clearly marked: an island far to the north, and a dwarven city that was pretty close by. They decided to go to the dwarven city first, and were surprised to find it occupied by drow.

Not that they get along with any elves, really.

Even so, they somehow managed to sneak inside and make their way to the castle, only to be spotted while scaling the gate. At some point while they were fighting off drow, giant spiders, and web golems I decided to throw in a dragon to mix things up. Not just any dragon, mind you, but something that I felt best represented what the Underdark had to offer: a purple dragon.

Now, I chose a purple dragon because they also lived underground, and I figured that drow could strike a bargain with one. Also I had never gotten a chance to use one before, and have the mini and everything to go with it. Unfortunately having never used them, I was not privy to a very specific weakness until several rounds into the battle, that being sunlight. When purple dragons are exposed to sunlight they react almost the same way as most vampires do, in that they can only take one action a round and take a fuckton of damage to the tune of 160.

Each round.

They are like the mogwai of dragons.

What should have happened was that the dragon should have instantly exploded, probably like the boss at the end of the first Blade movie, just to drive home how silly the mistake was. I ended up retconning the dragon into being a black one, dropping its level considerably as a concession since I could not conceive of a situation where a purple dragon would ever venture to the surface during the day. In retrospect I should have read more carefully (or just kept my mouth shut), but in all fairness this was the original 4E statblock format, so the weakness was mentioned almost at the very bottom.