The underscored line is the dangling article.  The three articles
immediately below this have negative numbers; here is a schematic
showing the article numbers:

a. (  23) M Harris: 215508
b. (  46) M Harris: -3
c. (  51) G T Smith: -2
d. (  23) M Harris: -1
e. (  23) M Harris: 215508

Line a is the dangling article, lines d and e are its other
occurrences.  When I again typed RET or `n' on line a, nothing
happened.  When I typed RET or `n' on line e, the cursor jumped to
line a.

After doing this I quit the group with `Q' and then reentered it
again.  Now the threading was no longer fragmented, but different: