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'Shameless' Set Panic After 18 Tests Come Back Positive ... Massive Lab Screwup

Phew! That's the operative word after "Shameless" had to deal with a massive COVID-19 outbreak scare following tons of false-positive results on set ... TMZ has learned.

Production sources with direct knowledge of the situation tell TMZ ... the set went into full panic mode after recent COVID tests revealed 18 positive results. We're told production feared it had a massive outbreak on its hands.

Our sources say there was a lab screw up ... and bottom line, those 18 positive results were not positive at all. This does not appear to be a flaw in the testing method ... we're told it was a lab error.

It's also unclear if production shut down to deal with what was initially thought to be an outbreak ... which our sources say went down in the last week. The show recently shot large scenes with as many as 200 extras.

We're told they had also been shooting on location across various parts of L.A. as well as on the Warner Bros. lot.

When production resumed, the show's executive producer, John Wells, revealed the pandemic would be weaved into the 11th and final season of the hit Showtime series.

This COVID outbreak scare ... it's real, unless the lab screws up.

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