It starts when OP posts a Mythbusters gif featuring a soccer ball being shot out of the back of a truck, titled Firing a soccer ball, at 50 mph, out of a cannon from a truck going 50 mph. In other words, cancel momentum.

/u/funintheburbs jumps in to point out that the concept being illustrated is relative velocity, Not momentum... spawning 58 children all trying to out-pedant one another.

Meanwhile in another thread /u/kclejeune similarly points out that the gif is about relative motion, and is set upon by /u/rabbittexpress who despite being wrong absolutely shuts down the debate proclaiming "You're so far away from understanding how the physics work I'm not going to waste my time. The video very clearly illustrates exactly what I have stated. I got the degree. You don't. Good luck.