When importing a large number of files, MC16.0.49 will report that it has failed to import a subset of the files, even though the files were indeed imported (they show up in the Recently Imported playlist and if I re-import the same files, it doesn't find anything new.)
This particular problem is repeatable (I can remove the files from the library and using ~d=r), and the same problem occurs each time. However, the exact number of failed imports are different, though similar, and the exact files that fail are also different, though perhaps similar. I say perhaps, because it's not obvious if there is a pattern.
I've seen this multiple times, but the most recent case, I'm importing 832 files (it doesn't have to be this many to fail, however) and I get ~70 files that are reported to have failed to import. If I save the list of failed files and compare it to the next run, I see that ~15 files are repeat offenders.