My worst mistakes involved homebrew riddles and puzzles. I realized very shortly that I just don't reason things out like my players, and what makes sense to me, and what conclusions I draw are not necessarily the same as where my players arrive at.
I hope this makes you feel better.
I have only run ONE session where riddles worked. Every other time, the players, who are the ones actually forced to figure out the riddle, either knew the answer right away or had no clue what it meant. In general, I have found that to be true in life. If you are in the right place and mindset for riddles, they aren't too bad. If you aren't expecting one, and get hit by it, they are not fun and usually bring the game to a halt.
As for my own blunders as a DM, here are a few. (I keep thinking of Red Dwarf's smeg ups and Kryten saying, "It's not that there are many. However, this is the first in a thirty volume set . . . " )
2E: I made an adventure that was supposed to be characters running away from a superior defense point, a tower with archers, a forced road to it and lots of traps and tunnels. The players assumed they could defeat anything so instead of doing the scouting, as I intended, they kept attacking and were frustrated they couldn't take the tower. I eventually had to tell them they were only supposed to scout it and the next part was them taking part in the siege. They didn't like that.
I had a huge battle for high (17+) level characters planned out and they looked at it, made some thoughts and then guessed where they thought the main bad guys were. They ended up blasting that place to tiny pieces with spells and items. It killed the main bad guy. I congratulated myself for making it possible to guess where the big bad was and on them for figuring it out. I figured they would be happy as the rest of the forces decided not to fight but it meant the climactic final battle was over after their first action.
4E: I am still getting the hang of the system. I created a fight for 9 PCs by having two dragons. Unfortunately, the dragons had too many hit points and high enough defenses that although the group was wearing the dragons down, I don't know what would stop the dragons from flying away. After two hours, and all of us tired at the grind, I ended it and faked my way to get back on track.
So, after the dragons, I made a fight with lower level monsters but then it was too easy as non minions couldn't take more than two hits before going down.
In fact, in general, it's weird how things work out at time. I ended up going with the full stats of a monster nine levels higher with some extras, and they nearly walked through it easily. The next session, I had a fight with 2x the party number with a more level friendly set and it almost overwhelmed them. A few bad die rolls by them and a few good ones by me could have reversed their fortune. (This is specific to 4E but fits most editions of DND, imo.)
I made a fight with Frost Giants that used some abilities to great effect. However, after they did it once, and it was something they could do at will, the players shut down the giants by dividing them with zones and walls and attacks. The giants never managed to pull that attack again and the rest of the combat, from my perspective, was a joke in that it was only a matter of time before the PCs won it.
World of Darkness: I had this really big idea. It is set in Chicago and I had the parties genealogy back twenty generations, including the ones who arrived in Chicago. The idea was to send them back in time to help their family, preferably by not interacting with them at all or not having their ancestors realize who they were. They would also help set up the modern vampire prince and get on his good side for the fight in modern day. It ended up being a big joke as none of them could take it seriously. What was supposed to last six sessions I killed in the same one I introduced it! Did not work. (If I could do it again, I would use the examples from DMG2 and do vignettes and have them play their ancestors. Alas, I got it three weeks later, too late to do anything with it.)
This is really only a small sampling of them. However, I have finally forgiven myself for any bad sessions prior to '95, as I was young and inexperienced.
edg