I've been lucky to fall in with a group with 2 good DM's. Now I'm running a homebrew, having never DMed before. So far, judgement calls gone wrong:
1. inviting an extra 3 people into the game (8 total) so there are always 2 or 3 people missing. Each player will only run their character, so there are 2 or 3 NPC's cluttering up combat. I can't see any one solution to this, but if I can I'll have those characters leave the party at the start of the session; if it wouldn't be plausible for them to find the party again, I'll have them roll up new characters.
2. having a summoning token summon a druidic bear who then (they discovered upon leaving the night's refuge) summoned a shark. Yeah, from Strongbad. Not terrible, I know, but I'm not shooting for a silly tone; I'll have to weave in the same seafaring bear again long enough to give him a plausible story, as well as decide whether all summoned creatures don't disappear or just some. Dizzamn.
3. saying "I won't track XP, I'll just let you know when you level up." I'll have to just contradict this now, I guess, before my sorceror gets any item creation feats.
Least I haven't pulled any really dumn sh*t, I guess.