I went to a really huge school, so my smallest class had 40-50 students, and the largest one was probably 250-300 students. It was a normal occurence to see people come in later, leave early, go to the bathroom, etc. Cell phones would ring almost daily. I had a FORTRAN prof who would answer her phone and hold conversations while she was lecturing!
Overall, it never bothered me though. Generally my profs only were distracted by the people in the front, or if people had to walk past them to leave early.
The cell phone ringing has to be the most annoying thing in a class. I always make sure mine is on vibrate.
My friends and I have been guilty of texting each other during boring classes. How else are we supposed to stay awake?