Chamberlain v. Skylink
The 2004 case in which a garage door opener maker tried to wield the DMCA's anti-circumvention ban against a cheaper remote, and lost.
Chamberlain sold openers with a rolling code, Security+, that changes the signal on every press. Skylink sold the Model 39 universal transmitter, which opened them anyway. Chamberlain argued that the rolling code was a technological protection measure guarding the opener's software, so Skylink was violating Section 1201 of the DMCA.
The Federal Circuit disagreed. Someone who buys a garage door opener is authorised to use the software inside it, and Section 1201 requires a link to copyright infringement. Otherwise any manufacturer could lock up the spare-parts aftermarket by adding a little code and a handshake. Lexmark went the same way for toner cartridges that year, and the question lives on in the right-to-repair movement.