Hi guys,
I'm using the mystery cloud 50a esc for my scratch build and have been facing really weird problems.
I initially started with a 4s and after testing all the controls the motor started jerking and the esc started smoking. I thought it was a battery issue so I changed the battery. Everything worked fine and the motor was turning too but when I moved the rudder stick to both end points the motor would again jerk and the esc would smoke. Anyone had any idea what might be the problem?thanks!

It seems to only support 2s and 3s, so it would have got damaged when you connected 4s by mistake at first.  Intermittent working does not mean "all izz well" with it

See if you can get someone to help with figuring out what component is failing, most likely it is something in the initial stages.