If there are 2 blank lines (line breaks) then the paragraph immediately after is uneditable (green striped overlay).
Confirm here: http://en.wikipedia.org/w/index.php?title=User:Quiddity/sandbox&oldid=560048320
and see thread here: http://en.wikipedia.org/wiki/Wikipedia:VisualEditor/Feedback#Paragraph_editing_disabled.3F.3F.3F
Version: unspecified
Severity: major