I just logged back in after a few weeks’ absence and noticed that all cards I sent (11!) on the same day back in November are all expired. 
Seems they did not even make it out of whatever USPS receptacle I dropped them in. I am so sad. And because I mail from many places, I do not remember where I dropped them.
I’m sad for the recipients. I’m sad for me - so much time, money, and effort gone.
Has this happened to anyone else?
This happened to me once, but I know they were taken out of the USPS receptacle as I brought them directly to the post office in my small, remote town in the Eastern Plains of Colorado. It was 4 postcards all but one to Germany, all sent and posted on the same day, and all 4 expired and 3 were never registered. I was very frustrated with this. The 4th one was actually to an address that was less than 40 miles from my home- thankfully I was able to connect with her and she registered the missing card. I sent her another card to replace the missing one. I was really upset as I put in a lot of effort and care into choosing, decorating and writing the postcard for each individual person.
This happened to me when I was on vacation once, but it was only about 5 cards. I don’t know what happened to them and I was really bummed. I’m sorry about that!!
It helps to know I’m not the only one it happened to. Thanks for the replies.
I thing our fancy postal machines eat them…
I thought missing postcards were not allowed to be asked to be registered, apologies if I am wrong.
To me as well, when we were on holiday in Cambodia 10 years ago. I sent 14 postcards to family and friends, and not one arrived. We left them at the reception of our hotel, and I think they never mailed them.
And two years ago in December a dozen Postcrossing postcards mailed from a postbox in Nuremberg all expired. It was frustrating because they didn’t count for the German December challenge. 
I didn’t ask for her to register the card, I sent her a message asking if she possibly received it (as we are less than 40 miles away from each other) but had forgotten to register it. She hadn’t received it at all and she said she felt bad it had gotten lost when we’re so close. She asked me for the ID number and said she wanted to register it, so I asked her if it was okay to send her another card and she agreed.
Yes. It happened to me in June/July 2020 when I went to my Sister-In-Laws and mailed from a small post office there, Two of then eventually arrived but the third is still missing in the mail.
We are not. supposed to register postcards that did not arrive, Some people say they will, but I won’t and I won’t ask people to register my lost ones. I have several postcards that were received over 100 days after they were sent. I suspect one was delayed because the Postcrossing ID was listed under the zip code and messed up the automatic readers, You can always send a replacement card and hope it arrives.
In our area; it has been on and off in the news that the master keys for the public mailboxes are out in the world. Periodically, the boxes are emptied or broken into by bad actors.
I’ve been hearing on the news about postal blue boxes (mail boxes that are supposedly secure) being broken into a lot lately by thieves looking for cash and gift cards. I don’t mail my cards from these blue boxes, but for the first time in over 3 years of Postcrossing, I suddenly have NINE expired cards on my “traveling cards” page-- only a dozen are traveling. And that’s not including the ones that drop off after a year has passed. The countries where cards have gone missing are numerous, including some in the USA. But it seems to me that the number of cards not reaching the intended destination has increased sharply in recent months. Given the ever-increasing postage costs, it takes some of the joy out of this hobby, at least for me. Has anyone else seen a recent increase in expired cards?