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That sounds like one of Confluences former horses, if I'm right. She likes to mark her horses that might make lethal combinations with a tattoo like that. I'm guessing that horse has a white gene or maybe a splash. :) If that horse is bred to a horse with the wrong genes, then there's a chance the foal will die as soon as it's born. It's called lethal white syndrome. :)
Which of your horses has the tattoo? We can tell you what combinations are dangerous, and there's a chart on the forum somewhere and the Facebook pages that will help you out. :P

One dangerous gene that was missed was Frame. Olw. This is the one that causes lethal white syndrome when a foal inherits it from both parents. The other lethal combinations are caused by and cause other things.

In addition, Splash 2 (PAX3C7OY) and Splash 3 (Sps3) are lethal with themselves and each other, although so far, they are both okay with Splash 1 (Sps). They can now contribute to the danger of the high white spotting mutations of Kit, I believe.