Bad idea (AD&D):
Setting foot inside the Tomb of Horrors. Period. The reason that module came with pre-fab characters in the appendix was because Gygax at least had enough of a heart to provide you with an alternative to sending your beloved campaign regulars into that sadistic meat-grinder.
On the subject of the baddest AD&D "adventure" ever, this article is hilarious and spot on (adult language alert):
http://www.somethingawful.com/d/news/wtf-tomb-horrors.php?page=1
bad idea #1 i was playing a svirfneblin (deep gnome) fighter/thief 16/16 in an epic level quest (back in 2nd ed. long before epic rules were ever published)...we came into a room full of crystal skulls with ruby eyes, like 10 of em...i had never heard of a demi-lich before, but i learned quickly that they dont like having thier ruby eyes pried out with a vorpal short sword by a thieving little svirfneblin. on top of the fact that i had 10 lv25 demi-liches casting various nasty high lv spells on me, i had an insidious EVIL DM who thought that he'd have a good laugh with my stupidity...long story shortened up some, i ended up in the nine hells, polymorphed into a pink bunny, "insane with lust," and soul-bound some demon...ahhh good times.....
etc #1 i had this player, great guy, good player, fun to be aroud, but he was also kindda slow, often intoxicated, and spoke with a lisp. he was complaining one night that he didnt have a magical battle ax...but he kept saying magical battle ASS...an i knew what he wanted of course but i kept repeating "so you want a magical battle ass right" and he would comfirm "Yeths i wan a mashical battle ASS" so i made him role a % die...pow he rolled a 1...so i gave him a plaid, steel hooved, fanged magical attack doney...a magical battle ass! he wasnt amused much but everyone else got a good laugh out of it and he ended up using that donkey for very long time and it became its own little NPC/player companion.