One of the most blatant simplifications Caesar entered into his accounts was the Germanic-Celtic divide along the Rhine. Going by his writings, the Celtic & Germanic world were separated by this river. Caesar's motivation here was political. All his actions in Gaul were justified by the alliances with Roman proxies, by it being 'in' Gaul. The Germanic tribes were the 'others', whom he had to keep out of Gaul, but he had no goals beyond the Rhine. Convenient.

In reality, this divide didn't follow a river but was more of a patchwork. Some Germanic tribes most probably did live on the canonically Gallic side of the Rhine and vice versa. What is notably hard here. Is that we can only base ourselves on archeology, and artifacts sadly don't talk. So we are quite certain Caesar is lying/twisting facts here, but it is very hard to get the real picture.