MMM v3 issue....

[BrianG]

Could be a dirty switch, or a bad connection on the switch or any wires. If it was the switch or bad power connection, the ESC would go through the arming cycle. If it was a connection issue with a motor wire, it would just come back to life (although you'd most likely get severe cogging if it was a motor wire).

Not sure how this affects the warranty, but I always replace the stock slider switch with a sealed toggle. Not only does a sealed switch keep dirt out, they generally are more resistant to being turned off with a hard jolt (as from jumping). The switch in the link is rated for 3A @ 125v which is FAR more than you need, but the heavier current switches seem to have a more solid toggle action.