For something so basic, I always find this very tricky to get right. Interference from global state can sneak in all from a lot of different unexpected ways.

One of my games allowed people to save replays and levels to the web. The online infrastructure, creation of the editor and debugging of replays dwarfed the time taken to create the game in the first place, making the game take over 4x as long as it otherwise would have. (Note the levels and replays list is only from the last day, never got around to doing an all time best.)

http://www.rocksolidarcade.com/games/stuntpilot2/