Romney backtracked on abortion, and that's pretty much it. I suppose you could count his position on a Federal marriage amendment as backtracking. He always opposed gay marraige, but he didn't see a need for a Federal marriage amendment until the Massachusetts Supreme Judicial Court imposed gay marriage on the Commonwealth by judicial fiat.
As this article points out, Fred backtracked on about 5 issues.
Objectively, Fred is a bigger flip-flopper than Romney, even if your perceptions don't match that reality.