Please provide processor/OS/ram (allocated and available)/Minecraft version/Java version.
Note that if you are using windows XP or have allocated less than 250mb of ram, this has got a very high chance of happening.
Yes, this was a feature implemented in 1.9
"It is also worth mentioning that solid blocks 1 or 2 blocks high relative from the activator rail are considered valid locations for mobs, unlike for players." This was changed in 1.9.3. It was a bug
What minecraft version are you using? What OS?
It's probably a lighting glitch. Try updating the blocks (using command block or pistons to change the blocks behind them (this worked for me))
Well of course, this even happens sometimes on Windows 8, and its only a year old. IT IS a bug with your computer, it might just only happen now because of updates,new java versions, etc.
/tp @p -732 11 -318 -140 50 (make sure you do it in a superflat world)