It's pretty rare but this does happen sometimes, there are threads if you search the Step 1 forum. The NBME does a re-check of scores on July 1st every year to adjust for changes in student performance over time, throw out bad questions, increase weight of good questions in their algorithm, etc. Sometimes they find a mistake and notify the student via email in mid-July.
I remember one thread where a guy went from like 240 -> 220 and was pissed because he had already set up aways for a surgical specialty.