Oh man, something awful happened to me today. I lost all my data on my DK64 cartridge! I got it used, and I never took it out of my N64 since I got it, until today. I had one completely finished file and two other ones I had gotten pretty far on, and after I took the game out today and put SM64 in, to make sure it did play when I had an expansion pack inserted into the system, I found that all the files on DK64 were EMPTY when I put it back in! This is how it happened: I was testing my duplicate N64 expansion pack to make sure it worked (I'm thinking of selling it), so I took out and put in my two expansion packs a few times. During this time I turned on DK64 a few times to see if the game worked properly after I inserted the packs. Then, after I had put the first pack back in, I tried putting SM64 in for the reason mentioned above. Then I put DK64 back in, and got a black screen when I turned it on. So I took it out and put it back in, or maybe just wiggled it. I don't remember if I had to take it out and put it in any more times to get it to work, but when I did, I found that all the files were empty. After the initial shock, I decided to see if the game would retain any new data. So I started a new file, and got the first banana, and then selected "Save" on the Pause screen. I turned the game off and turned it back on, and the files were all EMPTY again! I guess the game was already fairly old before I started playing it, and I admit that I did sometimes play for long periods, even though I don't know if that had any effect on it. I know I got WAY more than 50 hours of play out of it - maybe even 100 or more; it's just that good of a game. Fortunately, I have plenty of money right now (I'm saving up to get the new Nintendo system that I still can't get myself to say the name of), so I promptly ordered another copy off Half.com.
EDIT: Holy cow, that was long.