Here's something interdasting I found.

If you take the hex dump of a block like this one https://blockchain.info/block/00000000000001d58c445bfeb80caf313c5ba03be3ce7312714566852075b8ae?format=hex and search for this string 'ffffffff' you'll notice it comes up a lot. My guess is it's some kind of padding BTC uses between tx or something like that. Now if you look at this hex dump and search for the same string 'ffffffff' https://blockchain.info/block/000000000000005136f5e629aa02a2cda32b2adfe8fef42ff5572eb2978ae9e1?format=hex you'll notice there's a huge gap where that string doesn't appear. My guess is that's some kind of file that is contained in one large tx. In not sure if it's in the OP_RETURN or input scripts or what? Any help?