Constantly. They added a bunch of scripting extensions so a lot of fillable forms render a message telling you to use Acrobat until all of that stuff executes.

Those are probably PDFs with XFA forms. Not very common in general but if you often get PDFs from sources that use that then yeah you'll see it all the time. Good news it that it was deprecated, it's not in PDF 2.0.

Happens often if you're using documents that are signed with a smart card. Sadly, browsers can't sign PDFs yet either.