From @ticpu:

I tried the FS module and it was working just fine until I ran very busy timers. Then it wasn't able to read/write files anymore. I issued reset(); and tried again without any success, afterward I went to the computer, fsck'ed the SD card with no error and noticed a file that was in fs.readdir(); missing from the computer listing.
I tried again on the Espruino and the missing file was still appearing, I then tried readdir() without any SD card and it seems to list the same files over and over again ignoring the fact that there is no card.
I finally was able to recover by using the reset button followed by BTN1 to ignore loading from memory.
I did a quick check on the code jswrap_fat.c but I'm not at ease with JsVars yet, could it be some sort of static var that reuse the same space in memory?

http://forum.espruino.com/conversations/507/