The tires will fail as doozer says. THe belts can separate, this is not good especially at Z rated kinds of speeds.
You could inspire another cruciate memorial by the roadway.

If this was the case, there would be SERIOUS markings and warnings on the sides of directional tires - not just little tiny arrows. 20% of the time, the darn tire shops mount them backwards anyway...

When I worked for a racing team that raced in IMSA in the 80s, we used to test at the same track Firestone would do tire testing. The funny thing about their testing was the vehicle they were using. Ford E150 van filled to its Gross Weight with lead weights. We would take turns using the track. Usually 20min. per session.

Anyway, I asked one of the engineers this exact question. He told me that it makes no difference in the dry but in the wet it is a must for proper tire performance. He said they would test directional tires in the wet (mounted backwards) to make sure they were not dangerous. He said underinflating and worn out tires are the most common tire failures. He never saw a tire mounted in the wrong direction fail because it was being used backwards.

There is only one function of the tread pattern: to get rid of water. There won't be any dry traction difference.

The idea of a tire coming apart if driven backwards is a myth. While it may have been true 30-40 years ago, this problem has been solved long ago. I remember seeing a press release from Goodyear stating so.

I'm not a tire engineer, but the consensus seems to be that in the dry it doesn't matter. That said, we flipped the rears only at Willow Springs during the OTC, and were surprised by the diminished rear traction -- in the first session out. As they were driven more, they improved.

It's pretty common practice to flip them in track use. My suggestion would be to flip all four, though -- not just the fronts or rears.

If anyone has used Kuhmo Victoracers lately, they can attest to this happening. Apparently, these tires were directional at one time, but have since not been used as such. I'm not sure why, but the information enclosed stated to run them on either side as long as the thicker wear bar is on the outside. These tires don't have an aggressive tread to begin with and half way through there life, it is mostly worn anyway. Directionals are for water channeling anyway.