In the episode "Invention Convention", when Brackenreid is going to the dentist, actually physically being coerced by his wife of all things, Crabtree comes by, gives him the report of the status of the case, then runs off with the apple Brackenreid was gonna have as a reward. Later comes the Call Back from Brackenreid as he thanks Crabtree for him giving the report and running away from the dentist.
In the episode "Who Killed the Electric Carriage?", Inspector Brackenreid finds out that his wife (who's been giving him a hard time about his drinking) and other ladies from bloody Temperance League participate in bloody gambling, so he orders Constable Crabtree to arrest everybody in the bloody den. The following exchange has perfect comedic timing and is complete with hilariously ominous music.
Constable Crabtree: Sir? What? Even your wife?
Inspector Brackenreid:Especially the wife!
Inspector Brackenreid comments on one boy's poetry in his letters, and casually asks Murdoch whether he ever wrote something like that. Murdoch says that he's not much of a poet, but he recalls composing and being proud of a rhyming couplet when he was a young boy... on their family dog. Inspector's face goes from mildly amused expression to an extremely excited one, and then he just gets visibly disappointed. Murdoch's one-liner after the beat is priceless: "He was a very well-behaved dog."
Murdoch is explaining to constable Crabtree that he's using a liquid, testing whether there is blood on a bullet. If there is, the haemoglobin will cause the reaction. George nods and just repeats that yes, haemo-GOBLIN will do that. Oh, George...
Murdoch nearly drowns in a room on a boat that is slowly filling up with water. He is saved by Dr. Ogden after she and Brackenreid drag him out of the water. The first thing he says? "I lost my hat."
In the epilogue of "Murdochophobia," Crabtree is delighted when a previously-thought-dead spider that he grew a liking to throughout the episode, comes back to life. Brackenreid proceeds to drop off some papers for Crabtree to go through. Right on the spider.
In the episode "The Knockdown", the scene with the ferret as a bloodhound, especially when the ferret climbed up Murdoch's trouser leg.
In the 7th season episode with the tunnelling thieves, Crabtree assures a somewhat exasperated Murdoch that he knows "there are no vampires....in North America"