Is there a word for accidentally stealing something?

Is there a single English word to describe when someone has appropriated property that doesn't belong to them unintentionally?

For example, say I borrow a pen from someone and absentmindedly put it in my pocket after I finish writing. I discover the pen much later. Did I steal it, or is there a term with less harsh connotations?

You could probably call it an honest mistake (I know, a bit illogical, but still a common phrase).

Following the precise definition, if you borrowed a pen and then failed to return it as agreed, it would still be borrowed—that is took and used with the intention of returning it.

You could say you walked off with it? The OED defines this as:

to walk off with: to carry away as a prize or plunder; to steal.

but in present usage, it’s a bit broader: it can cover either deliberate or accidental stealing, depending on context. Quickly googling "walked off with" for examples, the second hit is:

$11000 Stolen Scarf Was an Accident. New York socialite says she walked off with it by accident…
    — NBC Connecticut

It is not only a question of harshness, but of meaning.

Even though you can find dictionary entries that will say (CALD)

steal
to take something without the permission or knowledge of the owner and keep it

where it seems that it does not matter how you took it (intentionally or not), in actual use "to steal" implies intention as per following definition (MW)

steal
to take the property of another wrongfully and especially as a habitual or regular practice

To say wrongfully implies "bad" intention and does not convey the meaning that you have in mind without further explanation.

I don't know of the word which does; all related words I looked at have this implication except the most basic and widest term: "to take".

• "acquired" (euphemistic)
• snarfed (regional/cultural)
• ended up with (too many words)