There have been a few excursions in the past where the pilot inadvertently advanced one throttle in forward while working the reverse levers on the other engines - although these have been on four engine aircraft, not twins (at least the ones I'm familiar with).
But it's at least a possibility that if the pilot didn't pull the reverser lever on the deactivated right engine, they inadvertently advanced the forward thrust lever on the right when de-selecting the left reverse lever.
A totally uncommanded thrust advance (Uncontrollable High Thrust or UHT in the lingo) is extremely rare on FADEC engines (something like 1/100,000,000 per engine flight hour), but it has been known to happen. It was more common with the pre-FADEC cable engines - usually due to a broken throttle cable...