I got off with a warning... asshole

On my first shift at my sweet, loving, paying job since returning from break, I walked in and...

Boss: "You missed a shift."

Me: "What?! No..."

Boss: "Yes, you missed a shift."

Me: "Okay, what one?"

Boss: "The Sunday after spring break"

Me: "Sunday was a PART of spring break."

Boss: "I sent an email."

Me: "Maybe I didn't get it."

Boss: "Geoffrey had to cover your shift." (During my shift, Geoffrey is always there, the whole time, simply sitting in the back. To do my shift, all he had to do was move to the computer at the appropriate desk)

Me: "Well during my shift I was at the AIRPORT. Not on campus. Spring. Break."

Boss: "Consider this your warning."