"Shit, I'm sorry," he says, automatically, without thinking. "I swear, I've been using this potion for two years and I never once had a problem. My affinity must've...must..." he buries his face in his hands. "Shit." He traded those potions to people he liked, people whose judgment he respected and whose trust he wanted to earn; now he has to tell them that he's all but betrayed them, in the first twenty-four hours of their time stuck in a deathtrap together.
(It does not even occur to him to try to hide the flaw.)
And here, in front of him, is someone he could have possibly befriended, who for the next five days or more will have a fucked up memory because he didn't do enough testing. Arguments like but ingredients are expensive and you used it yourself for a month before you even let anyone else try it fall flat, swallowed by the enormous hollow feeling in his gut.
Alexius takes a deep breath. "I...I have to tell the others. I'll reimburse you the mana, a...after."