Jack's deadpan warning to Ethan about the explosive gum: "Just don't chew it."
In the climax Ethan is nearly killed when he hooks the line from the helicopter Jim Phelps planned to escape in to the back of a speeding train as it enters the Chunnel and blows it up. When it's over the camera pans from Ethan on the outside of the rear car breathing a sigh of relief to a shocked train conductor inside who had witnessed the whole thing and promptly faints.
As the disavowed team is about to go into full silence mode for the Langley heist, Krieger sneezes loudly, causing their sound meters to briefly go haywire.
Ethan: Krieger, from here on in...absolute silence.
(Krieger nods, then gives his giant nose one last wipe)
The poor CIA employee getting hit with the sickness fluid. At one point during the heist, he emerges from the bathroom thinking it's over, takes two steps, and promptly runs back into the bathroom.
While Jack's upload of Sarah's info into the embassy database is taking longer than expected, forcing security to run it again, both Ethan and Sarah snark at him via I Have This Friend..., while staying in character.
Sarah: Tell me, Senator, do you all have someone on your staff named Jack?
Ethan: Jack... I believe we did have a Jack. An unreliable fellow as I recall. Constantly late and behind in his work. I was forced to tie him behind one of my best stallions and drag him around my barn for a few days.
Jack: Relax your crack, Foghorn. I'm working on it