The thing is, the reason always seemed to be “Could Not Generate Valid Parallel Plan” for most of them, even though more explicit reasons were available.
They started cropping up, as things do, in Azure SQL DB, and have finally made it to the box product that we all know and mostly love.
Let’s explore some of them! Because that’s what we do.
Also check out Rob Volk’s comment, as he lists out all the ones he could find, noting that most of these do exist in SQL Server 2019.