Years ago, I recall Zuckerberg being given a fab tour. It sticks in my memory because, at one point, he did something dumb like remove his face mask in the clean room, and that ruined like a whole batch of wafers.
He did indeed. Several tools had to be wiped down to get them operational again. Obama just strolled in wearing his normal street clothes.
The worst, however, I can remember is about 20 years ago. One of the vendor field service engineers ended up getting his hand stuck in a 1st gen copper electroplate machine. The robot had pinched his hand and held it in place about half an inch above a bath of fairly high-concentrate sulfuric acid. The other guy working on the machine with him was fairly new, and the only thing he knew to do was what was taught to him in training - hit the big, red EMO button. The EMO basically powers the machine completely down in event of an emergency.
So now this poor guy has his hand smashed by a wafer handling robot so close to an acid bath that if he rests his fingers they're going into a vat of boiling acid. And the machine is now unable to be powered up to manually move the robot with the teach pendent. He's stuck.
The fire department was called in. They ran into the cleanroom in just their normal firefighting gear and used a sawz-all to chop the robot arm in half and release the FSE. It took almost 2 weeks to decontaminate the facility after that one.
Mr. FSE was largely unharmed and returned to work within a couple weeks.