if you chain up as much execute commands as possible (like /execute @p ~~~ execute @p ~~~ execute @p ~~~...) and add a valid command (like say Hi!), put all that into a repeat command-block, activate it, it slows down some parts of the game. if you activate a pistone now, while it retracts, mc crashes with a missing block entity error.
get the crashreport yourself, its reproducable.