Read the linked comic, then take a read of the How it works post from a few days ago. The XKCD comic is basically showing a series of GET requests and what happens when the requested data size doesn’t match the actual data size.
XKCD’s take on Heartbleed
This reminds me of how on a DEC-11 in 1980 I learned that I could create a file of arbitrary size and it would be full of others’ old data.