It's an omen about the ugly rockwall.
poor rock wall, sky don't likey :facepalm
More informations though, what sky tex are you using, what sky is in your map description...
Are those people using a certan vid card, driver?
Although I'd assume theyd have probs with all maps, not just yours, so it's probably in the map.
try
sky_night_01
That's the tf2 sky. If yours isn't custom it must be halflife (2) in which case only people with half life would be able to see it. I got Orange box so I would see it, if someone only bought TF2 they wouldn't.
This can be due to low settings. Check your vtf file, try redo it using different program.