21.0.96 and 21.0.97: bizarre next-line behavior

[E. Jay Berkenbilt]

I apologize for this extremely vague bug report, but I cannot
reproduce this behavior reliably yet.  When I can, I'll submit a
proper report.

Since 21.0.96, I've seen several occurrences of C-n working normally
until the cursor gets to the last line after which it acts more like
return.  In other words, the cursor moves to the next line each time I
hit C-n unless it's on the last line in which case it actually breaks
the line at that point while keeping the cursor over same place as if
I had hit return instead.  C-p goes back up, then C-n goes back and a
second C-n continues to break the line.  Hitting C-e and C-a and then
doing more C-n seems to restore C-n to its proper behavior.  I don't
know what happens to get a buffer into this state.  Studying C-h l has
not helped so far.  I report this in case anyone can think of why this
may be happening.  In the mean time, I will continue trying to figure
out what gets emacs into this state.