During a crash the pucks push on each other with forces that are equal and opposite every instant (Newton's third law), so whatever momentum one gains the other loses. The total momentum can't change — not in an elastic bounce, not in a sticky splat, not ever.
Kinetic energy is different. A perfectly elastic collision (restitution e = 1) hands every joule back. Turn the dial down and the crash converts motion into heat, sound and dent — at e = 0 the pucks fuse and move off as one lump, losing the most energy a collision possibly can.
The restitution coefficient e is just the bounce-back speed divided by the approach speed. A superball is near 1; wet clay is near 0. It's why crumple zones save lives: they make the crash as inelastic as possible, spending the car's kinetic energy on folding metal instead of on you.
The gold arrows are each puck's momentum, m·v. The pale arrow at the centre of mass is their sum, drawn tip-to-tail — and it is the same arrow after the crash as before. The centre of mass itself glides through every impact in a straight line, as if nothing had happened.
Watch the two bars on the right. The momentum bar returns to its mark after every crash; the energy bar only holds when the dial reads ELASTIC. What a lossy crash gives up leaves the table as spray and heat at the impact point.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.