If you fail a part of bootcamp like the PFT or the rifle qualification, you get recycled back to another platoon until you pass which could make your stay in bootcamp longer which sucks. If you fail the PFT, you'll get dropped to Pork Chop Platoon where they try to get you in shape. That's why I told you before in the other post to get in the best shape as possible (especially your pull-ups and running) before you decide to enlist since Marine bootcamp is non-stop physical and mental activity. If you get sick or injured during bootcamp, you'll get dropped to MRP (Medical Rehabilitation Platoon) where you recuperate while at the same time you workout in the gym like Pork Chop Platoon to keep you in shape instead of slothing around the squadbay. You don't want to be stuck in bootcamp any longer than 3 months and you don't want to go to Pork Chop Platoon or MRP. Pray that you don't get sick when you step on the Yellow footprints for the first time when you arrive to the Depot from the airport USO. You have a certain number of chances when you get recycled back before they decide to drop you to RSP (Recruit Separation Platoon) and send you packing home if you fail the rifle qualification or if you fail the PFT and have been in Pork Chop Platoon for awhile. Or if you stay in MRP long enough (I think 2 or 3 months) if you got injured or sick, they'll drop you to RSP if you can't be fixed in time. However, you can request to go to PEB (Physical Evaluation Board) where some injured recruits stay for a few months longer until they decide you're physically fit enough to finish bootcamp. There are some PEB recruits there who have been in bootcamp for up to a year. However, you can only stay there for up to a year in PEB before they decide to send you packing home if you're not healed in time.
Check out this recruit training schedule from MCRD San Diego to see how it works: http://www.mcrdsd.usmc.mil/RTR/Copy%20of%20RTR_Matrix%201.htm
If you do get kicked out of bootcamp and you want to re-enlist again, it depends on why you got kicked out of there in the first place. If you failed the drug test, fail a background check (ie you forgot to pay your speeding tickets or you did not reveal to MEPS about a past medical condition), or do something stupid like go UA (Unauthorized Absence) during firewatch or try to commit suicide, you'll get an RE-4 code on your DD214 during SEPS class when you get dropped to RSP. An RE-4 code on your DD214 document means you cannot be able to re-enlist in the military ever again. Also do not do something really stupid like trying to commit suicide because every time you apply for a job in the civilian world, you have to tell your potential employers why you got kicked out of the military when you have to show them your DD214. All employers need to see your DD214. Also, with an RE-4 code on your DD214, you won't be able to apply for federal government jobs like the Post Office, US Border Patrol, and Secret Service. If you fail the rifle qualification or spent a lot of time in MRP or Pork Chop Platoon because you were injured/sick or you fail the IST/PFT, you'll get an RE-3 code. Which means you can get a second chance with a waiver but you have to wait around 2 years after your ASVAB scores and your medical checkup at MEPS expires before you can re-enlist again. Check these websites out to learn about the different RE codes in your DD214 when you get discharge as a servicemen or if you get an ELS (Entry Level Separation) as a recruit:
Military Separation Codes & Reenlistment Eligibility Codes
http://usmilitary.about.com/od/theorderlyroom/l/blcodemenu.htm
http://usmilitary.about.com/od/theorderlyroom/l/blcode3.htm
Upgrading Your Military Discharge
http://usmilitary.about.com/cs/generalinfo/a/dischargeupg_5.htm
Check this thread out and look down for the question posted by "leftout" and the answers given by Marine recruiters:
http://www.leatherneck.com/forums/showthread.php?t=5237
There are some modern Marines who went to bootcamp twice because they were in MRP, Pork Chop Platoon, or fail the rifle qualification the first time around but then they made it a second time around. Some older guys I used to work with at this aviation company who are former Marines/retired Marines and combat hardened veterans from the Korean War, Vietnam War, and in between those wars in the 1950's and 1960's told me that they wouldn't go to bootcamp a second time if offered a second chance again even though bootcamp back then was a 6 to 7 weeks and they used to live in those old quonset huts as squadbays like Gomer Pyle USMC near receiving building instead of those modern H shaped barracks.