The other thing is that many garage door openers have a ring buffer (first in, first out/erased) that only holds 5 devices, so if you program in more than 5 opener devices (ie: the original 2 fobs that come with the opener, maybe 2 cars with Homelink, a keypad), the first one that was programmed gets kicked out. I ran into that when I tried to add a 6th device for a friend.
-- mike