Volkswagen Passat / Audi A4
Historically, enthusiasts equate the VW Passat (called Quantum in the 1980s) with Audi's spectacular 4000 sport sedan (called 80 in the rest of the world). The two cars were identical except for their badges and the fact that the Quattro versions were wagon-only for the VW and sedan-only for the Audi.
However, in 2005, the VW split from the Audi platform. Alphabetical confusion arises because the A4 is now built on VW's B platform, like the Passat. These days, there are so many variants and differing lengths and widths of this platform, not to mention unique drivelines, that for practical purposes the cars are now merely step-cousins.