Stress fracture = going home, no questions asked, no matter how minor.
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I'd say generally that is true. However, there is an exception to every rule. I went through PLC-Srs in the summer of 2005 and had a friend in the platoon who was diagnosed with a stress fracture just before the end of week 3. I don't know who made the call, but he was allowed to stay and train and graduated with the rest of the platoon. Granted, he succeeded in creating a number of other fractures and has to wear an orthotic for the rest of his life, but he is now an officer in the Marine Corps. Just goes to show, not every injury will get you sent home.
The rule of thumb when I was training at OCS was that it had to be either a very grievous injury (see openbah's story) or something that prevents you from training. For some people, a stress fracture takes them out of the game. Evidently, others can train through it.
If you're in pain, ask yourself this: Am I hurting or am I injured? If you're just hurting, look around the squadbay: everyone is in pain. If you're injured, try to get yourself taken care of early enough that you can continue training. Just don't become a sick-call commando (going to medical for every little thing). The Marines and candidates around you will take note and that is bad news.
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