MMP reverse polarity

Hi,
I have my MMP reversed polarity with 7.4v lipo.
There was smoke coming out of esc.
Battery wire became stiff when cooling down.
However, the esc was back to life when I had correct polarity.
I could not notice any wrong with the esc in my 1/10 buggy.
Do you know where the smoke came from?
Do I need to send the esc in for further inspection?
Thanks.