https://www.looper.com/1549458/ncis-issue-sean-murray-mcgee-mainstay/

A small problem Season led to "NCIS" bringing back Sean Murray for a single scene, ultimately leading to the show adding Timothy Mcgee as a series regular.

An Editing Mistake Turned 1 NCIS Guest Star Into The Series' Longest-Running Character (https://screenrant.com/ncis-editing-mistake-timothy-mcgee-guest-star-longest-character/)

This editing mistake turned a season 1 guest appearance into NCIS’ longest-running character. The character will continue to evolve in NCIS season 21.