The fixed quantum resonance shared by all matter in a given universe, which cannot be altered and so reveals that Worf has been slipping between parallel realities.
The quantum signature is introduced in the Next Generation episode Parallels. Worf, returning from a bat'leth tournament, finds details of his life changing around him: a different cake, a different painting, a marriage to Deanna Troi.
Data scans him and finds that his RNA resonates at a signature unlike everything else aboard. All matter in a universe shares one signature, a constant that no known process can change, so Worf must come from another quantum reality. His shuttle passed through a fissure in space-time, and La Forge's VISOR has been triggering each further shift.
The fissure breaks down and hundreds of thousands of Enterprises pour into one space. The crew find the shuttle matching Worf's signature and send him back through to seal it.