Zathras says Sheridan is 'lost in time', Sheridan immediately chimes in 'and lost in S...' but Zathras immediately cuts him off 'Do not say, even now many issues with copyright'. A couple of minutes later when summarizing Zathras says 'not only is The One unstuck in time, and the thing... we cannot mention'. This is referencing the TV series Lost in Space (1965), which included a young Bill Mumy in the cast.
Zathras tells John Sheridan that Delenn will "never give up, never surrender" in her search for him, a clear wink at the catch phrase used in Galaxy Quest (1999).
At one point Zarthras says Sheridan is lost in time and then he stops Sheridan from saying lost in space for legal reasons. This not only reference, almost, the show Lost in Space (1965) but, "Lost in time and lost in space," is also a line/lyric from The Rocky Horror Picture Show (1975).
At the end, Young John Sheridan walks alongside Delenn through Babylon 5 and meets various characters. Delenn appears in her pre-chrysalis version (full Minbari, with no hair), as she was prior to season 2. This is apparently another alternate timeline, because in the main timeline Sheridan arrived at the station in season 2, and never met Delenn when she looked like this.