When Michael is showing Cameron around the school on his first day, he is introducing the different types of people. When he gets to the coffee kids, you hear one spill some coffee and say "that was Costa Rican, butthead!" As they walk away you can see the ground, and there is no coffee on the ground...anywhere. Maybe it is quick dry coffee!
True, you don't see any coffee on the ground, but the person probably spilled it on themself, or someone else. That's why there isn't anything on the ground...
I watched this film just after reading all the slip ups and I noticed that you don't actually hear any coffee hitting the ground so it might haven't not gone on the ground. It could have landed on something else.
umm listen close, you can hear the splatter of the coffee on the ground it would not make that splatter sound if it was spilled on the other guy. It is a valid slip up.