Une gaffe is a word that has two colloquial meanings. The first one is “blunder”. It sounds a little old-fashioned to me but you can still hear it today.
Je crois que j’ai fait une gaffe.
I think I goofed up.
Instead of faire une gaffe, you can also use the verb gaffer.
However, gaffe is much more common in the expression faire gaffe (note the absence of article), a very frequent colloquial synonym of faire attention, i.e. “pay attention”.
Désolé, j’ai pas fait gaffe.
Sorry, I didn’t pay attention.
Fais gaffe, y a une bagnole qui arrive !
Watch out, there’s a car coming!
Listen to the examples: