poor performance vs. overtasking

Apropos of a recent Dilbert it got me thinking of how certain problems can manifest themselves.

suppose an employee's performance is not up to standard. this would include the following symptoms

• excessive overtime
• missed deadlines
• poor quality of delivered work

on the other hand suppose an employee has been overtasked, assigned more work than there is time to complete. the evidence of overtasking includes

• excessive overtime
• missed deadlines
• poor quality of delivered work

well what else to say? it may not necessarily be 100% of one or the other. also since the symptoms are the same there may be very sharp disagreement between management and the employee what the real problem is.