I have seen this happen before and it is not acceptable.
I always allow 15 minutes in between each set. That gives time for the next act to be up and running and gives peopel a chance to go for a pee or drink etc.
Having an MC who controls the timing is so important.
I provide a printout of the timings for each artist to see, which shows clearly what time they are on etc etc.
It's not rocket science, just needs good preparation.