Spent a couple of hours trying to figure out a problem on hammer, only to find out it was a stupid mistake on my part, and it got me thinking: who else has made completely stupid mistakes? share them here! heres mine:
was trying to make some panels animate when i hit a trigger, but it wouldnt work. tried everyhting-renaming things, changing the animation i wanted, copying the outputs to a button, everyhting i could think of. then i realised i had been putting the animation names in the 'via this input' field, rather than making that 'set animation' and putting the animation in the 'paramater override' field.
So come on, can anyone top that?

Forgetting to add a arrival_departure_transition to a map.

Wondering why fog disappearins why looking into the sky, later realising had the sky set to black_no_fog

Wondering why the map said it had a leak when I had a brush entity shoved out in the middle of the void (couldnt see it because it was covered in nodraw and I toggled nodraw visibility).

Accidently hitting H when I had a bunch of stuff selected. Had no idea what happened...

Putting an instance inside itself while I didn't save a project.

chickenmobile wrote:
Putting an instance inside itself while I didn't save a project.

Instance-ception?

After hours frustration, I realize a trigger is not working because the flags are incorrect, or the filter is wrong.

Also, not saving before you open the texture browser. That can suck too...

Forgetting to save often and then Hammer crashes.

I had a func_brush that was initially disabled, set to be enabled when a named cube touched a filter. I could not figure out why it wasn't working, and tried all kinds of adjustments in an attempt to fix it.

Eventually I realized I had told it to Disable the brush, rather than Enable.

Forgetting to disable a trigger that opens and shuts doors.

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...

Having a leak in the map not knowing that rotation center of func_door was in the void

Spending a whole day, adjusting and fiddling with lights, just after the Portal 2 update, thinking they screwed up lighting in HDR, when actually it was because you placed a 2nd light_env in your map.... doh!

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.