Your credit card comes with Terms and Conditions.  You've never read them.  But they include, somewhere around page 18, a warning that if you make any payment late, the card issuer has the right, if it wants, to increase the interest rate that applies to your Outstanding Balance.  And it may do so retroactively.  Without notice.  

You make a payment late.  The card issuer, Chase Bank, raises your rate retroactively.  And you sue under the Truth in Lending Act.

Blawgletter knows what question just came to your mind.  You want to know whether that violates Regulation Z, right?

No, per a unanimous Supreme Court today.  The Federal Reserve's Regulation Z says, among other things, that credit card issuers must give advance notice of a "change in terms".  12 C.F.R. § 226.9(c)(1).  But, as the Fed conceded in an amicus brief to the First Circuit in a different case, the regulation doesn't unambiguously deem a rate hike that the Ts and Cs allow a "change".  Because the terms allowed the change, the Fed admitted and the Court therefore held, it didn't count as a change.  Chase Bank USA, N.A. v. McCoy, No. 09-329 (U.S. Jan. 24, 2011).

Although, since August 20, 2009, a different set of rules applies.  So we all have that going for us.  Which is nice.