Not to say the condition does not exist but I have never had a Captain Sim product barrel roll or turn out of control or anything out of control.
However, what I have experienced is on a long flight there have  been times when the aircraft (777) is following the flight route and then for what ever reason won't pick up a way point which results in off course condition.  When I check the FMC the way point is in fact in the FMC. This doesn't happen all the time but on at least three occasions during the last month while doing a long haul.