No, but in AVCHD it is. You can only record one or the other. If you format a card when you're in PAL mode, and then switch the camera to NTSC, it won't be able to record on that card. You can only record PAL footage onto a card that's been formatted by a camera that was in PAL mode.
*pedantric clarification: AVCHD is neither PAL nor NTSC, those terms shouldn't be used with HD, it should be "50Hz" or "60Hz".
Sorry I was busy a few days and did not respond. I had formatted the cards for PAL on a Saturday shoot and then a day later I wanted to format them back to NTSC and got the error message. Was worried I'd have to rent cards or buy new ones for the next day but after reformatting many many times it final took. I don't know why.