Answer
An adverse opinion is issued when misstatements, individually or in aggregate, are both material and pervasive to the financial statements—so pervasive that an 'except for' qualification would be inadequate. It states the statements do not give a true and fair view. It's the most serious opinion about misstatement, signaling the statements are, overall, unreliable.
Real-world example Because the company wrongly used the going concern basis while liquidating, the pervasive impact led to an adverse opinion.