In "Parallels", our Worf finds himself uncontrollably shifting into alternate "quantum" realities, one after the other. Eventually, the crew of various alt-Enterprises succeed in locating the prime "quantum" Universe, and hail "our" Enterprise to arrange for Worf to go home.
He's replaced the Worf in each quantum universe, and discussions with alt-Troi suggest a belief that when he leaves one, there is no Worf in that universe from that point on.
Notwithstanding that everything apparently reset to zero at the end of the episode (so those alternate Worfs presumably poofed back into existence), who was the Worf on the bridge of the prime Enterprise during the hail? Where did he come from?
If this was also "our" Worf, that's very confusing because now one consciousness is apparently being copied across Universes, which makes one wonder why you'd bother sending "traveller" Worf anywhere at all: he's a duplicate and, much like Thomas Riker, has no right to the life of the "prime" Worf.
On the other hand, if this was some alt-Worf, had he poofed into existence to fill the gap? Or did he come from an alternative "quantum" universe, too? Did he not notice the changes? Nobody on the bridge of the prime Enterprise seemed to think there was anything wrong with their Worf, though they were also suspiciously content to receive a new one.
What's going on?