Students are sorted by last name differently in Assignments than they are in the Gradebook (and Roster and Grading), so it is easy to enter grades for the wrong student unless you notice the switch.

Assignments: lastName, preferredName
Gradebook (et al) : lastName, firstName, then preferredName (even when the firstName is not displayed)

For example, in Assignments:
Smith, Bob
Smith, Jim

But in the Gradebook, they appear in a different order:
Smith, Jim
Smith, Robert (Bob)