It's been pointed out that Livery would be broken if there were ever a cost-increaser, since gaining a Horse with Livery would trigger another Horse, which in turn would trigger another and so on until the whole pile was gained
But I just recently realized that if you also had a Trader in hand, it would be even more broken - because you could reveal the Trader to gain a Silver instead, which would also now be $4 (or more if there were multiple increasers in play), which would trigger the gain of another Horse that you could reveal Trader for and so on, ending up gaining both the entire Silver pile and the entire Horse pile. Changeling would have a similar broken interaction
Now, I'm wondering - would there be a way to fix that if a cost-increaser existed (let's call it Anti-Bridge)? One option to resolve the "gain the Horse pile" problem would be to put an Experiment-style clause on Livery "gain another Horse, which does not come with another Horse", but that would just look weird in the vast majority of games without Anti-Bridge (especially bad if Menagerie itself didn't have Anti-Bridge). Another interesting possibility would be to make Horse effectively the opposite of the variable-cost cards - there'd be a below-the-line clause stating something like "this card's cost cannot be changed", so that both conventional cost-decreasers and Anti-Bridge would leave it unaffected, which could have an odd interaction with Remodel-type cards - Remodeling a Horse would get you increasingly expensive cards the more cost-decreasers you have in play. This would also automatically solve the Changeling interaction, since Changeling is only exchanged for, not gained
However, that would still leave the broken combination with Trader unaffected. The new version that was announced a while back that uses the exchange mechanism instead of would-gain would resolve the Trader interaction, which is a bit ironic since obviously that change had nothing to do with this hypothetical problem. But even then, anyone playing with a pre-errata version would still have the same problem. But, since it would require three different cards (Livery, Trader, Anti-Bridge), it would almost never actually happen