Why Fuel Injectors leak - failed O-ring replacement?

The only O-rings I’ve ever replaced for injectors (had that problem w/my VW Rabbit) is the one that seals the injector where it seats. That one prevents air from leaking past the injector and into the engine, causing a sort of vacuum leak condition. That o-ring isn’t what prevents the injector from leaking gas when it is turned off, only to prevent air leaks into the engine. Are you replacing O-rings on the injector’s pintle-operating parts then OP?