I fondly remember spending 2 weeks completely disassembling Sonic the Hedgehog once, I was always curious how games worked. Disassembling them turned into a little hobby of mine -- you always find leftover graphics, dead code and sometimes even whole unused levels when you poke around, it's lots of fun.

If you're interested, up-to-date versions of Sonic the Hedgehog disassemblies can be found here: http://info.sonicretro.org/Disassemblies

People have also disassembled other games like Super Mario Bros., etc.