Slips:
Ss know the correct form, and make the slip due to carelessness, tiredness, distraction, inattentiveness, etc. Ss can usually correct these kinds of mistakes themselves
Errors:
Ss make a genuine mistake because don’t actually know the correct form. The error is made due to factors such as trying to produce something which has not been fully learnt or learnt incorrectly, interference from L1, mistaken beliefs about the L2 rules. Ss are not usually able to self-correct errors