It is very common in Rome Total War: Barbarian Invasion. I often played as the Western Roman Empire because it was so hard to win.

Once when I was playing as them on hard I lost, to the Saxons… They didn’t even really beat me.

What happened was France revolted and became Western Roman Rebels, then the Huns invaded while I was at war with the Berbers and the Eastern Roman Empire in Africa. The East declared war on me, and then I took Alexandria (that city is a victory condition for the Western Roman Empire, so why not). I had shipped over much of my forces to Africa and was winning on all fronts. I even eventually took that Berber city out in the middle of the Sahara and defeated them.

I retook Ravenna from the Huns and then I got the notification “the Romano-British have been defeated.” I thought… ok hmm that means the Saxons took all of Britain from the Celts. One of my diplomats was near the Rebels capital in France and the Saxons were laying siege to it. I was like “Well, the Saxons will take my rebels out, and I can remain neutral with them for now because it is fine. They'll be weakened and then I can retake Gaul.”

Three turns later I got a defeat notification with images of defeat across my screen.

It turns out that the Saxons have a hilariously easy victory condition: take Britain, take Samarobriva, hold Saxon territory+14 other territories.

I did not know that.

After that game I never abandoned Britain. I never let the Saxons take it again.