Stupid Hammer Mistakes?
Forgetting to disable a trigger that opens and shuts doors.
My Work
[spoiler]Maps:
Revenge of the Angry Turrets
Capture the Cube [Co-op]
Capture the Cube 2 [Co-op]
TPWEGTH Sample Map
Aperture Aquatic Testing Center
Aperture Aquatic Testing Center 2
Aperture Time Testing Center
ML's Halloween Trick - 1000 downloads!
ML's Halloween Treat
ML's Combination - 1000 downloads!
ML's Jailbreak Labyrinth
ML's Tricky Teamwork [Co-op]
WIP:
"Capture the Cube 3"
Workshop Maps Link: http://steamcommunity.com/profiles/76561198008890579/myworkshopfiles/[/spoiler]
Setting a func_brush that has it's solidity toggled to always solid...
Forgetting about texture lock and rotating all the textures individually after rotating a giant group of brushes...
In my first map I didn't know about face edit... I didn't have any nodraw textures (brushes were textured uniformly) and had crazily bad brushwork that made me not be able to compile...
[spoiler][SP] Alternate[/spoiler]
Having a leak in the map not knowing that rotation center of func_door was in the void 
(percentage calculation sphere self-test failed!)
108.1% done, -8.1% remaining...
Released here!
All maps for Portal (1/2) worth playing are mirrored here.
I've had multiple cases of accidentally duplicating entities (in the same location) without noticing it.
This is especially fun with triggers, relays, env_entity_makers and such.
And in earlier versions of Hammer it was possible to delete all of the brushes in a brush entity (in "solids" select mode), leaving behind a brush entity with nothing in it, which is completely undetectable in Hammer and causes mysterious and random crashes in-game.
It literally took WEEKS to figure that one out.
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic