there's a bug in the standaone mode which allows you to be invisible. Well, gotta play deathmatch now. cya P.S I'm reporting it.

You mean when people turn invisible?
I'm not sure what you're talking about but I'm guessing it's when people turn invisible at the start of a round. They're probably lagging or what not or maybe you're lagging.

At deathmatch I've noticed people have turned invisible. This is mainly due to lag, not hacks.
I'm not sure if this answered what you were telling us but like what Bolinboy said a little bit more information would help.

Invisible mice are also quite common on some vanilla maps such as 63, 64 and 65.