It happens sometimes in the wild. I don't know anyone (other than myself) that is attempting it in captivity. A pacific gopher x cal-king hybrid found in the wild:

And another wild one, scottsdale, AZ mix of king and gopher:

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6.7 pituophis c. annectans
1.1 senticolis t. intermedia
1.1 rhinechis scalaris
1.2 lampropeltis m. thayeri
2.1 pantherophis g. guttatus
1.1 lampropeltis t. campbelli

1.1 turbo corns
1.1 jungle corns
1.1 thayeri hybrids (het albino)