Employees can have slump periods, times when they're not firing on all cylinders. What is the top trait you look for to determine whether they're simply having an off day or if something more serious is going on?
An off day, off week is fairly common.
I live by the old adage of "inspect what you expect." This basically means, when worrying about an off moment, go back through and check out prior performance.
Character traits generally don't happen over night. The same "mistakes" will have been prominent for weeks, months, leading up to a manager noticing it.
If it is a sudden decrease in performance or quality, ask about it. Ask if they need additional training. Chances are, the employee knows they aren't performing and may give indication as to why, so a mutual solution can be found.