whatever you do, DO NOT lie to your boss

This is the worst mistake ever. Poeple can tolerate mistake, imcompetency but not lies.

Can you think of a way to make it sound like an innocent mistake like you forgot sth instead of you intentionally lied about sth? Without detailed circumstances it is hard to come up with excuses. Of cause this also depends on if your boss wants you or not. If he is looking for reasons to let you go, then that's it. If he wants to keep you then he would be willing to accept your excuses even if they could be lame.