This is a list of some common issues and potential fixes that happen in TF2. They may be server specific, but most of the time it's just TF2 being buggy.
"Couldn't CRC map maps/MAP_NAME.bsp, disconnecting"
This happens when a map you're trying to download failed somehow, usually by a partial download.
*Downloading files from our downloads page downloads them in a .bz2 format, which you'll need 7-Zip to open.
Please see the above fix for "Couldn't CRC Map". It requires the exact same steps to fix this problem too.
In your console, copy and run this
record demo; stop
That should fix most problems with missing players
In your console, copy and run this
snd_restart