Take a look at the original sentence again:
Mike has terrible eating habits when he
opened his lunch bag, we saw a bag of potato chips, two candy bars, cellophane-wrapped
cupcakes, and a carton of chocolate milk.
You wanted to fix it this way:
Mike has terrible eating habits, but when
he opened his lunch bag, we saw a bag of potato chips, two candy bars, cellophane-wrapped
cupcakes, and a carton of chocolate milk.
A perfectly good method of fixing a fused sentence is to separate the two complete sentences with a comma and a coordinating conjunction like but. However, when you use this method, the revision must make sense. But is a contrast word, and the two sections of the fused sentence above are not showing contrast.
You might want to consult the rules for fixing comma splices and fused sentences.
Go back to the sentence to try again.