What could happen?

Article VI
Running Conduct

6.1 Permitted Conduct. A participant must run or walk the entire portion of the run course. Except for reasons of safety and when no advantage is gained, all runners shall follow and remain within the prescribed course. A participant who gains forward progress by crawling or otherwise violates this Section shall be disqualified.

If your sugars are too low, you could have a seizure.  You could also experience some brain damage from the heatstroke or hypoglycemia.  I can't think of a race where I would think it is worth it.  Of course, I am BOP, so it may be different for those who are in the very front.

Potentially...the bonk you describe is scary and usually temporary, but what if you fall over and end up with a concussion (even with a helmet) or broken bones?  "Pushing through" muscle cramps can result in muscle tears, strains, tendon issues, etc.  Your endocrine(?) system is fairly robust, but you can seriously throw internal organs out-of-whack by pushing too hard for too long (kidney and/or liver damage as your body tries to deal with muscle waste).  I'm not a doctor, but go poke around some of the ultramarathon and/or adventure racing sites and you'll see examples where people pushed too far. 

• Hyponatremia can lead to death.
• Rhabdomyolysis can lead to acute kidney failure.
• Compartment Syndrome can lead to nerve damage and/or muscle death, and potentially loss of a limb.
• Cardiac arrest can be caused by pushing too hard.
• Heat stroke can cause death or life-long neurological problems.
• Vigorous exercise can lead to blood clots within the musculature, which can then lead to pulmonary embolism or stroke.
• Pulmonary edema can result from fluid imbalance/kidney failure.
• Hypoglycemia can lead to convulsions, dizziness, loss of consciousness, and coma.
• If your body can no longer regulate its temperature, you could become hypothermic, leading to convulsions, loss of consciousness, coma, and death.
• Less extreme but still serious, physical exhaustion can render you unable to perform basic daily functions until you return to health.

First, it depends on the race.  If you are doing a sprint or Oly, you are talking about 1-3 hours of activity.  It is pretty hard to really bonk unless you are really under nourished.  Yes, you can have heat stroke or heat cramps, but even that is not super common.

As for a HIM or IM, it is more likely.  Also , to push yourself to that extreme is very hard.  Pushing through pain vs pushing through an injury are two diferent things.  Pushing through pain is not necessarilly bad, pushing through an injury can lead to permanent issues.  You need to know your body and learn to understand what it is telling you.

Example - last month I ran a 1/2 mary.  At mile 10, my left quad starting locking up.  I could tell it was a cramp and not a tear.  So I stretched, walked, pushed through it till it loosened up and then finished running the last mile. 

I was sore for a couple of days but by the next weekend, I was able to run again.

If it had been a tear, I could have been risking months off or worse.

Learn to listen to your body.  As you get better at this sport, you will know when your body can't continue.