"I didn't know that." (2)

Another reason why "I didn't know that." cannot be an acceptable excuse:

In many cases, it is not "I do not know" but in fact, it is "I refuse to accept the fact (even though I am aware that the fact might be valid) for some other reason."

And this, your conscience knows. So, even if you try to cheat your conscience, usually it doesn't work, and you will find yourself unhappy.

You cannot cheat your conscience even if you could cheat everybody else around you.