The moment in Ptolemy's Gate when Ptolemy dismissed Bartimaeus even as they were attacked by twenty djin, and Bartimaeus trying fight the dismissal spell to stay and protect him. Same goes for Nathaniel doing the same.
Kitty being willing to cross over into the Spirit World for Bartimaeus.
For many, it's a Tear Jerker whenever Bartimaeus talks about Ptolemy.
The Cerebus Retcon present when rereading the trilogy, in Amulet of Samarkand, Bartimaeus only mentions Ptolemy in passing as "someone I had once loved." It comes back to bite in Ptolemy's Gate.
Miss Luttyens trying to defend Nathaniel from demons - happens offscreen, but it still counts.