To slip up is to make a careless mistake. Slip up on the dates.
It implies carelessness rather than ignorance, which is why it is easier to admit. Anybody can slip up once, and the phrase forgives it in advance.
1phraseslip upTo make a careless mistake.
“He would slip up on names and never on figures.”
slip up is a phrase.
Practised in Phrasal Verbs