I have had file numbering problems with my FZ20. I emailed Pana this morning and received the following email from them this afternoon::
"THe file numbering system uses what what be thought of as an index file. If the user simply reads files from the SD card to the PC all is fine. However, if the user writes or copies to the card from a PC the index system can get corrupted. I suspect somehow your PC wrote something to the card and that's where the problem originates.
I suggest copying all photos to the PC (either using card reader or USB camera cable. After all is backed up, format the card. From that point, ONLY copy from SD to PC, but never back to SD card. The file system used by DSC's is similar, but different to that used by your PC. hardware or firmware wise, I've never had any problems with this operation. Also, we saw the index system malfunction when very slow SanDisk cards were used, but they were 32 & 64 MB cards that were very slow."
Never never write to the card or delete files from the card outside the camera.