Take another look at the original sentence:
You wanted to fix it this way:
To add a comma between can and then would create an equally bad problem, a comma splice. A comma splice occurs when you have two main clauses joined with a comma alone. My cat Buster is always tipping over the garbage can is the first main clause. Then he searches through the trash for milk jug caps is the second main clause. The spot between can and then needs a stronger break than just a comma.
You might want to consult the rules for fixing comma splices and fused sentences.