It happened around 10 p.m. Thursday inside the store located on the 7200 block of Germantown Avenue in the city's Mount Airy section.
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Police say a 41-year-old man walked into the Wawa and became upset he wasn't being waited on quickly enough.
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He started yelling at an employee who was unpacking boxes and stacking shelves.
Police say the customer pulled out a knife. They say the employee used a box cutter to defend himself and stabbed the man in the chest.
The 41-year-old man was taken to Einstein Medical Center in critical condition.
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Police say the employee will not be charged since he was defending himself.
The Wawa has been closed following the incident. It reopened just before 6 a.m. Friday.
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