When you’re driving the Beetle in chase cam, the interior is completely white, and the front bumper is permanently deformed, as though it’s already been wrecked. This is most noticeable with the second bumper option.
Also in chase cam, the Ford F100’s bed has big chrome triangles where there should be screws.
EDIT: After playing more, I’ve found some more mistakes/glitches.
1949 Mercury Eight Coupe: When in motion, the wheels (not the hubcaps, but the actual wheels behind them) turn from black to chrome. This only applies to the manufacturer color.
1957 Chevrolet Bel Air: Same as the Mercury, but it also applies to wheels that have been painted.
2014 Ford Fiesta: When in motion, the rim of the wheels turn back to silver if painted. This happened in Forza 5, as well.
2010 Fiat Abarth 500 esseesse: See Ford Fiesta. Shift light is still nonfunctional, as it was in Forza 5.
1994 Mazda MX-5 Miata: See Ford Fiesta.
2011 Ford F-150 SVT Raptor: Window tinting applies only to the rear side windows.
2014 Tesla Model S: Window tinting does not apply to the sunroof.
1995 Volkswagen Corrado VR6: Window tinting does not apply to the front passenger window.
2013 Hyundai Veloster Turbo: Side mirrors have carbon fiber on the side closest to the car itself. This was also in Forza 5.
1971 Plymouth Hemi 'Cuda: Rollcage is covered with black and grey stripes.
1970 Datsun 510: Decals are cut off at the edge of each side (applying a decal to the edge of both left and rear will leave a gap, for example.)
2011 McLaren 12C: Manufacturer chrome paint removes decals on the painted surface. This can be remedied by using the “aluminum polished” paint with these options: Hue: 0.00, saturation: 0.00 and brightness: 0.90.
That’s all I can think of at the moment.
EDIT: Found another.
1984 Volkswagen Rabbit GTI: The “sport” brake upgrade makes the brakes invisible everywhere outside of the garage, including photo mode.