After sales slumped in the 70s they created a disastrous advertising campaign which is a case study in customer alienation: effectively 'drink Schlitz or I'll kill you' :-
Advertising didn't kill Schlitz. They made some processing changes to their formula that caused a micro infection. Not sure, could have been Pediococcus. But they did it all at once, and ruined so many batches, that customers left and never came back.
Oh, Schlitz: How a Historic Ad Campaign Helped Kill America’s Biggest Beer Brand