Feel free to confess anything you like. It can be a fun or serious transgression - whatever floats your boat as long as there's at least some truth to it.
I'll go first: I've always wanted to go to confession and confess to the priest that I've been having impure thoughts about him. I'm not Catholic, and I don't know any priests, but the temptation and mischievous impure thoughts are there.
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“Les grandes personnes ne comprennent jamais rien toutes seules, et c'est fatigant, pour les enfants, de toujours et toujours leur donner des explications.”
— Le Petit Prince