ive found a realy big bug in the 13w41a, if you pause the game entities look to be paused but when you unpause it the entities all move jump to a new spot, also happens with items traveling through hoppers and redstone, when you pause then game the look to have stopped then a massive surge of items flow through hoppers, its had to explain but if you do some fast tests you will see the problem
Duplicate of MC-36461- If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.