Alternatively, if you're on sv_pure 1 or something, there can be a conflicting file, which has been for me in the past.
Not actually mapping for the first six months because I didn't understand how to make brushes. Until I watched this (especially the scene where he hits the enter button)
Today I learned why you use the search function to see if a map name is already taken. I made a koth map called Undertaker and uploaded a1 to the bot for testing. When the imp started I joined in, heard someone compliment the map and stated that I made it without leaving spawn. Then I noticed that things looked different did a quick run around and realized that I didn't make the map. Unknown to me there was an existing koth map called Undertaker in a1 and that version was accidentally hosted instead of the one that I made. In short I claimed a map I didn't make as my own because I felt a rush of pride when I heard a compliment. I'm naming everything in Latin from now on after I kick myself in the ass a few times.
After three maps I have only just now started using visgroups to organize things. And even then it was only to hide one section of clipping while I was texturing some stairs. After that I kinda... Forgot about it. Whoops.
Thinking that the skills I learned in Portal 2 hammer easily transition to a multiplayer shooter running an older source version. Or that the compile times would be just as quick.
Testing the wrong version of my map that works and ending up releasing a map with a broken cart. More than once
I managed to make a map edit entirely pitch black if you went into HDR. It was caused by a minor typo in the HDR scale when compiling but you get the idea!
always seeming to break compilepal because it could never pack my maps correctly, reinstall after about 6 months of not using it, works perfectly fine...
Deciding to a symmetrical map like Probed and wanting to make a tunnel between the two sides or taking 20 minutes to release I had 3D textures of
If you mean that you accidentaly removed the VisGroups Panel, you can easily enable that again. While a map is loaded, click on Tools, then Screen Elements and then check "Filter Control".