i know what you mean, that situation kind of puts your heart in your throat.
I wite down, email and photograph my ressie #'s once made so that i don't lose them.
Last year, when we made our ADR's we had one left over and couldn't decide before we left where to use it. while there (during Christmas, of all times) we found the only place we could eat on christmas eve was Akershus, 6pm, princess dining - we figured that we'd give it a shot, the conceirge desk at POR set it up for us. when we got to the desk at the restaurant @ 5:30pm the waitress couldn't find our ressie, she looked up with our name, phone #, and she looked at our hotel acct - she couldn't find it. the only thing she found was for 12/25 @ 8:05am, i told her that wasn't it - that was for CRT, she said "umm, oops, hold on, here is the phone number for reservations, you need to call there immediately", i told her that i made that ressie the day before and it should be there, she said that she accidentally cancelled the CRT ressie

. that's when the heavens opened up and it poured rain. she directed me over to the nearest pay phone to call (i wanted to object - she screwed up, she should fix it - i wanted to stay and argue this, but it came over me that if she really couldn't help me, then i could lose the CRT ressie if i didn't move fast enough), the phone was in the doorway to the ladies restroom (not a place i felt very comfortable being). i spent 1.5hrs on the phone, not because i wanted or even cared about akershus at this time, but because i didn't want to be charged for cancelling a ressie, she was able to make a new ressie for me, but ultimately i had to speak to the mgr of CRT to make sure i wouldn't be charged for cancelling - CRT worked out fine.
Turns out the ressie at akershus was made under the name "Guest, Riverside", grrr. But being 7:30pm and everyone soaked (except me, i was the guy on the phone in the ladies room), no one was in the mood to eat there.
That experience really soured me for quite sometime, i didn't want it to ruin the trip, but if we had lost the CRT ressie, it probably would have.