That is how a friend of mine lost a horse. It got a really nasty infection from a splinter and was put in a sling. It developed pneumonia and died from that.
I was warned long ago about something some horse owners don't think about by the vet for the US Equestrian Team, about transporting horses long distances. He said if traveling long distances [something the Olympic team does a lot] in a standard straight or slant trailer with their head tied, it was vital to stop them at least every four hours, unload, and let them get their head down and graze, simply to avoid pneumonia. He said a common enough mistake was for people to take breaks, let the horses out of the trailer, but then turn around and feed them from a hay net still while tied short to the trailer on the ~outside~.
He said the equestrian team now travels in box stalls on a truck, much like home. Transporting long distance is much easier on the horse if they can move around in a box stall or stock type trailer. I thought it was interesting.