Linka, the odds are increased if kept in small groups without proper male/female ratios, but it can happen with any two mouthbrooders.
I think the biggest problem these days is that many people are housing them together, and selling the fry/juveniles to the LFS without even considering that the spawns might not be pure. They can grow up to look just like either species, and you might not realize there is a problem until later on down the line when you have a clutch of fry that just aren't right.
Yellow labs have been overbred in this area for a very long time. Everybody wants them, so supply meets demand. That doesn't mean that the "suppliers" were responsible with their breeding practices.
I had a group of what I believed to be pure tank raised Yellow labs - absolutely stunning specimens...I never had any reason to question their lineage until one clutch of fry from them (after a few seemingly perfect clutches) turned out to be obvious descendents of red zebras...But I didn't have any red zebras! (Half the clutch looked like perfectly nice Yellow labs, the other half looked like perfectly nice estherae...)
I've never seen a male red zebra that couldn't prove dominancy over a male yellow lab.
I have seen male red zebras that thought they were entitled to breed with every species in sight, even though they have their own females.
The LFS are full of "all yellow" yellow labs (no black on the dorsal), too orange yellow labs, and even those that look perfect like mine, but prove to be hybrids.
These two species are very common...You can find them in any big box store (chain store) and any LFS in this area. The colouring is very alluring to beginning hobbyist, and they are pretty cheap compared to some. They wind up in alot of tanks with inexperienced owners who think the fry are cute. Then, they can eventually wind up in the LFS, or your tank...
I have a friend that has a colony of wild caught Yellow labs, and the little ones from those will be the only Yellow labs I ever have again!