CHICAGO (CBS) — The day before Thanksgiving is a popular day for parties and underage drinking, but in Lake County authorities are trying to stop the dangerous “Black Wednesday” celebrations before they start.

CBS 2 Morning Insider Lauren Victory went inside a secret stakeout.

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In Beach Park, a gas station clerk was busted for selling alcohol to a minor who was working undercover last October.

Another underage buyer entered the same store 13 months later. She was a plant, too, but staff didn’t fall for the trap this time.

“It’s always good to do it before holidays,” said Lake County Sheriff’s Detective Jakub Klatka, who leads the sheriff’s alcohol compliance checks three or four times a year. “We still have a decent amount of failures in these things.”

Karla Osario, a 20-year-old confidential informant on her last operation, described the steps she goes through in the alcohol compliance stings.

“The person at the register will either ask me for an ID. If they do, I present it to them,” she said.

Osario uses her legitimate Illinois driver’s license, which explicitly shows she’s under 21. She’s instructed to tell the truth when she tries to buy alcohol during the compliance checks.

“We’re not out here to make people fail this, or do anything where they would be in a situation where they feel like they’re being tricked,” Klatka said.

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Yet, detectives said they find blatant disregard for the law.

“Sometimes clerks or employees will bypass that system, and bypass the fact that they’ve looked at the ID, and still make a sale anyway, which is a problem,” Klatka said.

On one mission, Karla got an ironic response at Ma and Pa’s Liquors in Beach Park.

“He’s like, ‘I should call the police,’” she said. “I just walked out.”

No booze bought in that exchange. That’s how all of Karla’s tests ended; 23 out of 23 stores passed their compliance checks.

“It’s good to know that people in my community are actually doing their job, and everyone in the community is safe as well,” she said.

“It’s a good feeling to have when you do these compliance checks, and you find that stores are complying,” Klatka said.

Unfortunately, that 100% compliance was short-lived. The sheriff’s department busted two stores the next night.

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The Lake County Sheriff’s office also has partnered with Stand Strong Coalition for a new program called Tip411. Parents or youth can anonymously reports where drug use or underage drinking is taking place by sending a text message to 847411. The message should start with 123TIP, and then the tipster can provide as much information as possible on the issue they’re reporting.

Lauren Victory