If buying old games is a disease...

[A Black Falcon]

I lost my Wipeout 64 save once, soon after I got it, but the card was okay because that was years back and the card still works fine.

A few times my N64 cards have had issues and have partially erased themselves -- once I lost Goemon's Great Adventure (soon after I had re-purchased it and was about to get the last ticket... but I replayed it. Twice more, because it's awesome. :)) and a few other less important files (Excitebike 64 tracks, a Gauntlet file or two... (I have like 10... :D)... but as I said, I never had the whole cards erase themselves. I have one Nintendo brand card and one Performance (Interact) 1x card.

Gamecube... once my SpyHunter file corrupted itsself, and once the Midway Arcade Treasures file corrupted itsself (darn you Midway!), but other than that I've had no problems, and those corruptions didn't damage other files on the cards... All three of my cards are first party (59, 251, and 1019, but I don't use the 59 anymore).

I have never lost data on a memory card yet, official or unofficial. *Knocks on head* Both of my GameCube memory cards that I leave in the system are Interact cards.

You will eventually, if just because your N64 card's batteries die. :) ... unless you back up the files to your PC with a Dexdrive or something similar, that is.