“Just this once” is how every bad habit introduces itself.

Nothing ever introduces itself as a habit. It arrives as an exception, with reasons attached and an end date implied, and the reasons are usually genuine the first time. What makes it work is that a single instance is unarguable, so there is never a good moment to object. By the fourth exception the phrase has stopped being said out loud and the thing is just what you do now.

Count rather than judge. If the same exception has come up three times, it is no longer one, and that is worth knowing without any drama about it.