"I slipped up by sending the wrong email to my boss." "I slipped up because I forgot my friend's birthday." "I'm really nervous about the presentation... What if I slip up?" "He slipped up by saying, 'I hate you!' to his girlfriend." "She slipped up by saying that, huh?"
Slip up is a slang term for "mistake/error." The phrase is a noun, but if you want to use it as a verb, you would have to change "slip" to "slipped" or "slipping" depending on if you are using past or present tense.