Posted this on Tilt Forums, but I'll also post here.
I had a single ball trooper multiball. Here are the steps that I think can recreate this state.:
- have mummy multiball ready
- have third lock lit for trooper multiball
- graze captive ball to start mummy and collect third trooper lock simultaneously.
This will start a single ball trooper multiball. No additional balls are launched into play. You are stuck in this multiball mode until you drain your single