I just unlocked a treasure chest and got a card in it. The card was listed as Sloth from Fullmetal Alchemist; however, the character depicted was Lust, not Sloth.

Sites:
Secret Agent Fangirl Manga - https://safg.deviantart.com
Other art: quamp.deviantart.com

Erm, was the error on the actual card? Cuz if it was it was my fault.

As mentioned in my guide, there is an error (or was error) with card deletion. So, even if you delete cards, they sometimes (or always, dunno), remain, if the set is published. So we know of it.

The light of spirits is bursting forth within you! Indeed, your power has grown!

I've updated the cards code ( finally ) and removed any cards from the system that were meant to be deleted.

That one should be gone now, along with 183 others.

私は日本語を少し読むと書くをできます。あなたも？

Erm.... I don't think I'm making myself clear here. Perhaps a link to a screenshot should show you what I'm talking about. Also this was just taken, so the card is still there.
Link

Sites:
Secret Agent Fangirl Manga - https://safg.deviantart.com
Other art: quamp.deviantart.com

But the character on the card (at least in your screenshot) is Sloth. The two homunculus do look somewhat similar, but that's definitely Sloth. Lust's homunculus mark is at the base of her neck, not right above her chest like Sloth's.

She's right. That's really sloth.