At my last job, the stress was so severe that eating would make me get sick (yeah, it was THAT bad) so for a couple of years I didn't even eat lunch, but previously I would sometimes end up eating something else but it wasn't very often.
I'm usually pretty good at eating what I bring with me, but sometimes I just have a craving for something or other from the food court (I work in a mall). We've got a refrigerator, so I usually just eat the packed lunch the next day when this happens.
When that happens I just leave my lunch in the fridge at work and eat it the next day. Where I work we order take out most Fridays, but every now and then we do it on another work day too. Some times I have to say no to ordering out because I messed up my budget by buying an expensive toy.