It is possible to slide sideways off a tamed horse without a saddle. It depends on the orientation, but pressing the space bar sometimes results in going sideways or forward instead of backward.
MC-16887
Jumping Sideways on an Unsaddled Horse
Resolved
Fixed
0
Confirmed
bar, horse, jump, sideways, space
Snapshot 13w21b, Snapshot 13w22a, Snapshot 13w24a, Snapshot 13w24b, Snapshot 13w25a, Snapshot 13w25b
Snapshot 13w26a
Retrieved
Confirmed.