In the theater, not only is it considered rude and bad form, but there are an increasing number of shows (including everything Scott Rudin produces, I believe) at which, if you leave your seat, you're not allowed back into the space.
I just don't think that's okay. I suffer from IBS, and usually have no issues attending theater, but unfortunately a few times, I had to go to the bathroom, or there would have been a pretty gross situation. The folks at Groundhog Day, Hamilton, and Moulin Rouge were incredibly kind, and escorted me back to my seat when it was an appropriate time. If there a true emergency situation, and if you must use the bathroom, is it really fair to punish someone and not allow them back in their seat? I should be allowed to get up if the situation warrants it and not be denied my seat for the rest of the show.
It's so interesting to me that people who abuse the no cell phone policy constantly, through out the show just get warnings or annoyed looks. They don't get kicked out, even though they should. I am respectful theater goer who would never dream of taking my phone out of my purse, is it really fair to be punished so harshly over a medical issue?