The balance stop spring, often known as the "hack lever," is one of those components that is easy to overlook and can easily ruin a watchmaker's workflow.

Hack levers stop the balance when the crown is pulled out, allowing the time to be set precisely to the second. Since this stops the watch entirely, it's not the best choice for isochronism (other brands have levers that instead reset the seconds hand to zero when the crown is pulled, instead of stopping the balance), but it's the standard mechanism in the industry.

They tend to live low on the mainplate, below the bridges, and aren't actually crucial to basic functionality tests, so they can be easy to overlook in a hurry—and that can really bring a service to a grinding halt. 

Since the bridges are assembled atop the hack lever, forgetting to install it means uninstalling a lot of components to correct the error. The 2824 would require you to remove the automatic module and the barrel bridge, which is irritating, but not a showstopper. 

On a more complicated watch like the 7750, however, the hack lever lives below the entire chronograph module and the 3/4 bridge for the geartrain pivots. Forgetting to install the lever before this point essentially means you'll have to re-service the entire watch.