It happens, it happened to me this year. I had above average scores, grades, etc for the field I applied to (which is a non-competitive field to begin with). I was given excellent feedback "emails about how lucky programs will be if they get me etc". And on interviews had multiple positive comments on my LOR's, one even said that one of my letters was the strongest they'd ever seen.
But I made some mistakes, I limited myself geographically, my top 5 out of 9 were all very very solid upper-tier programs, and I didn't want to play the "send text messages and emails to the residents" game. Looking back, the fact I didn't match doesn't shock me anymore.
My bottom 4 or 5 programs got essentially no follow-up from me, and probably thought I was either not ranking them or not ranking them high enough to waste a high rank on me. My top 3 I have to assume just filled with candidates ranked higher than me on their list. I wouldn't be at all surprised if they filled in their top 10 to 12 as two were very geographically attractive to a good chunk of medical students, and the other might be the single strongest program in the country for procedural training in this field. And I know they all interviewed lots of folks.
To add insult to injury, I cancelled an interview last minute in early January because I was feeling (falsely) confident I'd found my top 3 programs and didn't think this place would change anything about that. Well, they had one spot in the SOAP and if I'd just gone to the interview and ranked the program I almost certainly would have matched, even if I was dead last on their Rank List. Yeah, the SOAP interview with them was awkward.
All that being said, the SOAP was extremely stressful. It took me a full week or so to recover my ability to sleep more than 2-3 hours, I didn't eat much for about 3 days, and I had a nearly constant tension headache. But I had a lot of interest from programs, about 20 different programs interviewed me by Tuesday night. And I got an incredible spot that all told I'm actually extremely happy and excited about. I even had one program that had only one spot to give away, and they called me on Wednesday morning before the first round of offers went out saying they were struggling all night between me and one other candidate. They had decided to send the offer to the other person, but wanted me to wait till the end of the offer acceptance window to accept any offers because if that person declined them they were going to send the offer to me in the second round. They promised to call me the moment they found out so that if they had the offer for me I could consider it against any first round offers I got. I wouldn't have taken it over the offer I accepted in the first round, but they did follow through and notified me when their first round offer was accepted.
So while it was stressful and a bit of a surprise that I didn't match, things worked out really well for me. I couldn't imagine myself being any happier had I matched in my top 3, and I'm certainly relieved that I'm going where im going rather than to one of the places in the bottom half of my ROL. In the end, it worked out kinda nicely. So if you're a good candidate who's from a U.S. school and haven't failed to match or been dropped from a program in years past, it can work out ok, even perhaps better than one hopes!