VALPARAISO | A Wal-Mart employee was charged with theft after police said he used a malfunctioning self-scan register as his own can't-lose slot machine.

Police said the employee discovered that a self-scan register was dispensing $20 bills when it should have dispensed $1 bills.

So, Valparaiso police said, he spent the next several hours making purchase after purchase -- essentially using the machine as his personal ATM.

Police said he made a total of 10 purchases that got him about $600 that he shouldn't have received during a four and a half hour period Tuesday morning.

The employee, Christopher Sheets, 24, of 3101 Black Partridge Lane, Valparaiso, was arrested Tuesday evening. Formal charges of felony theft were filed Thursday. He faces up to three years behind bars if convicted.

A store security official told police the problem began when $20 bills were loaded in the $1 bill slot, and $1 bills were loaded in the $20 bill slot of a self-scan register.

At 7:30 a.m. Tuesday, Sheets purchased a chocolate milk using a $5 bill and expected to get $2 and some change back. But he got $40 and some change. He kept buying more things throughout the morning, police said.

The trouble wasn't discovered until hours later when a customer ended up on the losing end of the deal, getting $1 instead of $20, and reported it. Security footage then showed Sheets using the register.

Police said he admitted doing it because he needed the money. Police said getting the wrong change once could be considered an accident, but repeatedly doing it is considered theft because the money was intentionally taken.

Police said they don't know how many customers got $20 bills and didn't report it.