Originally Posted by fimo
I've always logged into SQ for OLCI and keyed in my LH M&M number with no problems. And I even credit in and outbound flights to different accounts successfully. Logging into the website using your KF number is not the problem here. At least not in my experience.
I got bit with it once and found out because I happened to glance at my boarding pass as it got printed out. I made the guy tear it up, delete the SQ FQTV from the booking and put my LH FQTV number in.
Apparently both the numbers (your KF and LH M&M) get stored in your booking if you login using your KF number and checkin with your LH number. So once you check in, if the guy printing your boarding pass slips up you might get the miles on your KF account without your realizing it.