goof

Nope, good catch.

Throughout these movies, and others of the era, there was less attention paid to continuity than became the standard just a few years later. Especially when something is inconvenient, they just didn't worry about it. An example in another Thin Man film leaps to mind. Asta has been brought into the seafood restaurant on a leash, and through the evening the leash is handled by a couple of different folks, or is seen dragging behind Asta.

But at one point Asta chases a suspected bad guy out of revolving door, and then runs around in the door for a few turns. As Asta enters the door the leash vanishes, as it would obviously create a challenge for the "stunt" to come - running around in the revolving door, doubtless getting caught in the space and strangling our dear hero.

Of course, when Asta once again leaves the revolving door, and returns to the restaurant, the leash reappears.

Whether for convenience, or because eagle-eyed attention wasn't paid, these sorts of things just aren't uncommon. In fact, in the same scene as they're crossing the bridge, you'll note that as Powell changes lanes he seems to collide with a couple of the cars in the filmed backdrop.

Speaking of goofs, one of the user comments mentions the scene on the "Golden Gate," but the bridge is actually the west span of the San Francisco-Oakland Bay Bridge. So Nick is driving the correct direction (east) on the correct bridge to get to the racetrack, Golden Gate Fields on the shore of San Francisco Bay a bit north of Oakland. The Golden Gate Bridge would take him to Marin County, and they ain't no racetracks in that there neck of the woods.

All interesting observations. I noticed the leash issue as well. Depending on the production and the director, continuity varied. Given that director WS Van Dyke was known as "one take Woody", continuity was not a paramount concern. It was getting the job done. Powell and Loy worked well under Van Dyke's system.