It's happening quite a bit more now than I can remember in recent history.
Googoogaagaa (trotting bred on dam side, pacing bred sire)
Doc's Boo Boo (pretty decent Ohio bred trotter by Downbytheseaside out of a trotting bred dam)
Plus the horse that won the Elitlopp a few years ago (Don Fanucci Zet) is by a trotting sire out of a full pacing bred mare.
And on the flip side, saw a 2yo Long Tom win tonight in the Buckeyes at Scioto - 2yo pacing colt from a trotting bred mare. 53.2 and looked pretty good doing it.