If someone made an expensive mistake while doing you a favour, would you cover the cost?

Yes. A good deed shouldn’t be punished, and nobody should regret or suffer for having been kind.

Yes I would.

Yes, I would, then if she felt really bad about all that, tell her she can take you to lunch or a movie or something so she doesn’t feel too bad about it.

Yes.
Shit happens, and the example that you mention is purely bad luck.
Could have happened to all of us.

I’d insist on paying for the locksmith then I’d be more careful next time!!!

How much richer is she, than you.

What did the OP do that she has to be more careful next time @LadyMarissa?

Yes, I’d insist upon paying.

I’d feel it was my responsibility to not require an expensive solution to a simple problem. Unless I had told her deliberately, “I only have one key to my house, and I have no way to break into my own house that isn’t really expensive, so be sure not to lock yourself out!”

e.g. I probably would use another key, or have broken into my own house in a free or less expensive way, rather than pay a locksmith for that. Or use my home warranty to mitigate the cost.

It seems like a problem the homeowner was going to face sooner or later. The person walking the dog had no reason to expect they were risking the owner having to pay a lot for a simple mistake and being expected to pay for it.

If the mistake were less reasonable and did serious damage that wasn’t liable to happen, then sure that’s them really messing up in a negligent way that they could be considered really responsible for.

Yes.

That is something that I would have to decide on a case by case basis, judged by factors such as severity, and the degree of negligence.
Having to call in a locksmith is one thing, but if he burned my house down, because he decided it would be a great lark to have a campfire in the living room, I would extract every single penny from him.

I love you Raggy!

She should pay.

She was doing her sister a free favor.

Homeowner should pay for this one. Sister had good intentions and it would be nice if she continued to do favors in the future.

How badly do you need the money?

The more important question is.. Do you ever expect a favor from that person ever again?

If I was doing you a favor and made a easy mistake and you came after me for 100 bucks.. Well, walk your own dog dog next time.

Causation is that the lock is at fault, not her use of the key. Thus it’s the homeowners responsibility in this scenario.

Thank you, everyone. You reinforced my feelings, and I paid that bill. My sister is still saying she wants to reimburse me for at least half, but I’m going to try @canidmajor‘s idea of redirecting her to a lunch date or something similar.

Some additional info I probably should have supplied from the start:

I believe my sister feels especially bad because the dog-walking was sort of circumstantial. Her dog was at my house, so she would have been over there to retrieve her own dog even if mine hadn’t needed a walk. We regularly swap petsitting, and while I really appreciate every time she takes care of my dog, I think to her it’s less of a favour and more repaying me for the maaany hours of dogsitting I did when her dog was young. That said, she always goes above and beyond making sure my dog has lots of fun, and I was really grateful she took him out because it allowed me to go on a lovely trip.

As to the door, @Zaku: I have several keys. It’s just that when there’s a key stuck in the lock, the door can’t be opened from outside. That’s pretty common around here – my sister knew it would happen and called me while I was still out to say I would likely need a locksmith. Since all the windows would be expensive to replace, I’m pretty sure I found the cheapest way to break in. I like your general point of gross negligence versus simple mistake though.

@flo and @stanleybmanly This bill is probably equally inconvenient to both of us. Not fun, but certainly not a tragedy either.

@longgone If it’s equally inconvenient to both of you, then you share the cost equally.
As an aside the best thing is to have the old style doors. No such problem as locking oneself out.

By the way how much is it?

I shall take this opportunity to tell you about a babysitter I had in the 80s. Her name was Jennifer and she and her parents were members of my church.
I hired her to babysit one night while my husband and I went out.
We got home to the babysitter eas going “Look at your microwave! It’s not as bad as it looks!”
Well the child made microwave popcorn but didn’t follow the directions and walked away.
Popcorn caught fire.
.
Obviously that bag of popcorn was history and the babysitter controlled that event.
So she started over. Started a bag of popcorn and walked away again…
.
.
.
SHE DID IT TWICE IN A ROW!!
Yep. It DESTROYED my microwave. I called her parents. They steadfastly refused to take any responsibility so I just let it go.

As I side note: Does this mean you have a double-keyed dead bolt?
If yes, I’d change the locks. Here’s why:
Are double-keyed deadbolt locks safe?