Continued a test 13w17a map on new snapshot, I was there about 3 minutes spawned a horse, tamed, used saddle, flew a short distance, made a workbench, made bale of hay then clicked about trying to make a lead using string, there was a short slowdown/freeze while I searched for other possible items in the inventory and then window closed.

13w17a was working fine.

Will try to recreate tomorrow (3AM now)