i have a set of rules about what they can buy already since the fact is to towns and villages even citys they visit dont have the resorses to enchant items if a player wants a specific item they can buy it but it is usually a very under powered version cause i give out plenty of good stuff tailored for my party... they just have to be lucky enough to find it an example is
i have a paladin who wants a holy avenger (can you name one that dont) there is a rusty sword laying on the floor that he over looks just so happens to be what he's looking for just didnt make the insight to determine that fact as such left it where it was. had he picked it up the rust would have worn away and he would have a pretty new sword to play with...
did i mention im not the nicest dm in the world but my theory is if you look hardenough for what you want you have a better chance of obtaining said item

Be a bit more wary in what twists, events, chases and traps you put in front of a particularly devious player playing a particularly devious class. In retrospect, I shouldn't have allowed him to 'try out' the "Swordmage" Forgotten Realms class. And be a Githyanki.

He could pretty much just teleported enemies and allies alike through any obstacle, and used "Telekinetic Leap" at every oppurtunity to bypass obstacles and such. It took me a while before I could actually throw reasonable, yet realistic, challenges at what I have found to be a vastly overpowered class (in terms of out-of-combat usability).

Yeah. You win this round, clockwork demise. But your cunning will not save you from what is to come!

why didnt i think of that when i was playing the one shot adventure my brother put together will definatly foil his evil plans then hehehe

I was running a Star Wars group who were working for the Republic. We had 2 soldiers, a jedi, and a scoundrel. One of the soldiers was a clone. He found a little bbit of info on Grievous' past ( the part about when the jedi wrecked his speices ability to fight after they defended themselves) and they flipped over to the confederacies side. Everyone followed him. I meant just to shake their loyalties to make the clone not shoot the jedi in order 66, not to flip sides!

In the early stages of a the tabletop serenity game I played at uni the GM set the odds 10 to 1 for a boxing match the groups unarmed combatant was put in... the player won. The group won big bucks.

The GM did not learn however, future odds were all between 2 to 1 and 10 to one and he didn't put a cap on the maximum amount of credits the group could bet. Not quite sure why. The upshot was the group had hundreds of thousands of credits and we just purchased the coolest, fastest, toughest ship we could, put loads of big guns on it, and then flew around blowing up alliance stuff.

PS: It wasn't a serious serious game after the first session, it started off serious but instead of the GM correcting for his mistake he threw the campaign out the window... another big mistake if you ask me.

Not really a blunder so much as just a ill planned decision combined with a bad roll that should have been kept a secret.

I had my players in a dungeon that had a trap that made the ceiling fall as they tried to exit the room. The ENTIRE group rolled above 17 so of course they all got away, except the NPC that I had serving as the group's healer until one of the other players could get his work schedule fixed.... I could have fudged the roll but one of my players saw the die and started laughing. >.<

He stopped laughing when they ran into the next monster and several took heavy damage.

Worst and funniest blunder ever was a GM of mine playing rifts, he couldnt sleep for some reason cant remember why and at 3 am, my Character walks into an Inn and ask's "how much for the inn" GM char told me 100 credits, so I paid him and told the NPC to get out of My inn.