The kind of feature I’m thinking about is something where someone has the authority to make some change to a coin’s state, but does not have total and sole control of that coin. For example, if Plasma Cash were to implement any feature that enabled “vaults”, the hot key would have the ability to move the coin into a “pending move” state, but that move could be cancelled by the owner of the cold key. (The details of the vault and its timeout aren’t relevant to this—just the idea of one key that can update a coin’s state in only limited ways).
Suppose in the first diagram above, the coin in block 1 is controlled by Bob, and the coin in block 2 is a vault where Bob has the hot key (but Alice has the cold key, and thus would be able to cancel any move). Bob (with the cooperation of the chain operator) could jailbreak this coin from the vault, by double-spending the coin from block 1 to himself (creating the coin in block 3), and then spending the coin from block 2 to move it into a “pending move” state (creating the coin in block 4). Under your CFCR, he would then be able to withdraw the coin from block 3.