Following the Crossing of the Rhine in 406 AD by a mixed group of barbarians into the Roman Empire, there was a period of instability as northern Gaul succumbed to chaos and pillaging by the barbarians. This in turn caused the rise of three usurpers in Britannia. It also didn't help that the Crossing was a significant moment in the decline of Rome, due to it occurring at what was one of the WRE's most secure borders.
So, what if it never happened, or is just a failure? Perhaps there were garrisons in place at the time who managed to ward off the intruders. What would the (perhaps long-term) effects and ramifications of this be?