Luckily not suffered badly over many yrs cycling.
Worst time was c10 miles from home on a 50 milers some yrs ago, when the hollow feeling in my stomach suddenly took on cavernous proportions, legs went extremely heavy, and most worrying was my peripheral vision darkened. Luckily I was riding with my brother in law, and took his wheel at a ridiculously slow pace. Wasn't just a case of him taking the wind, but I couldn't have coped with watching the road, traffic, junctions etc. if I'd have been alone I would have had to stop. I simply locked my strange "tunnel vision" on his back wheel and grovelled.
When I got home, collapsed on the floor in the garage, my wife came out and asked me why my lips had gone blue? The darkness of the garage, produced some interesting strobe light effects across the peripheral darkness, and knowing I was home and able to simply sit slumped against the wall/floor, the unbelievably light-headed feeling was almost pleasurable!
Sugary pint of tea, chocolate bars, bananas, and felt slightly human again. Subsequently ate for England. Never put myself in that position again though.