I was just riding my horse (tamed long ago, wearing a saddle and a diamond armor), then I saw chunks that didn’t render. Since I’m not fond of stepping on invisible blocks, I figured I’d quit (while still on the horse) and re-enter my world to reload everything. It worked, but my horse mysteriously disappeared. As it’s an SSP world, I can’t really provide more details.
A horse of mine already disappeared last week (in 14w26c), but this time I had access to the commands. /say @e[type=EntityHorse] printed one horse, although the UUID provided was “badly formatted”, and the same command would give no results at all after a few seconds in the game. Leaving the game and reloading the same world would allow the commands to detect the horse for a few more seconds.