A missed birthday. A forgotten anniversary. A milestone that goes unnoticed. These small slights from a manager may seem like no big deal, but new research from Wharton reveals that even the mildest mistreatment at work can affect more than just employee morale.

The study found that when managers at a national retail chain failed to deliver birthday greetings on time, it resulted in a 50% increase in absenteeism and a reduction of more than two working hours per month. The lost productivity was a form of revenge, with slighted employees taking more paid sick time, arriving late, leaving early, and taking longer breaks.

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