I was testing out the horse breeding and when i tried to breed a horse with a saddle with another one that didn't work. So i spawned another and didn't put a saddle on it, then tried breeding it with the other horse without a saddle and it worked. Also tried it with no saddle but armor and it also doesn't work.I haven't tested this on a server but this just seems to be unimplemented so i imagine the bug would be there too.
Update-13w17a
Now it only doesn't work with armor. Like it says in the comments you can breed two horses with saddles only although I recall not being able to it doesn't matter because it seems to work now.
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is duplicated by 6
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Comments 5
Confirmed, but only with horse armor.
If one of them wears an armor, breeding fails.
Two horses wearing saddles can breed.

appears to be fixed in 13w17a.
Happens in 13w19a
This works as intended. The bug is that you can't remove armor from horses, but that has been fixed.
I think it might be intentional