"I'm going to need about 600 bits of entropy for this. Can you go the store and pick up some playing cards for me? Let's see, six hundred divided by log-base-two fifty-two-factorial—yes, three packs should be enough."
(Later, opening them ...)
"What the—!?"
log(52!)/log(2) = ~225 bits per fully shuffled deck. Remember, you want the log of the number of possibilities, which is not 52.
You're right. (This is embarrassing.) Updated.