I had it happen last year..FREAKED ME OUT!!! ...the entire terminal melted off..The reason..I had a loose , rusty , strany , frayed wire running to one of the terminals...

Thus the motor was starving for amperage...thus she got hot and started sparking,arcing and getting so hot it welded the wire to the battery..and the terminal burned completely off.Walmart gave me a new battery..SInce then I update ,my battery clamps when needed..