This cartoon is based on the double meaning of slip.

Slip as a verb means to lose balance and perhaps fall, especially on a slippery surface like ice.
“Be careful of the ice. You might slip.”
“She slipped on the wet floor and broke her ankle.”

As a phrasal verb, slip up means to make a mistake.
“Make sure you don’t slip up again.”
“The company slipped up with their actions.”

The maintenance team slipped up (made a mistake) because they had not kept the rink’s ice smooth.

A rink is the place you go to ice-skate.