Here's another "fun / NOT!" trap I found myself, just the other day....

Many of us choose to use the "Use Data Sel/Seq from a Section" System Function - to copy one sections Data Selection to another.... Until recently, I never had a problem with it.

Then, the other day I appended some Internal Data Selection to the Parent Section - Using Event Rules (Set Selection...).... I noticed, that the Subsequent's Selected Data did not match that for the source (WTH???). Once I copied the Source Sections ER based Data Selection into the Child - Hell Froze Over and things were all Play-Nice, again!

Now, I hadn't experienced this before - so I don't really know if it was 'just moi' or some random trantrum from the God of Nano - so, your mileage may vary.