So while mailing between alts I have occasionally typo although autocomplete has fixed that since it arrived.
Most of my mistakes were minor, a few mats or a lame transmog piece. But back in like WotLk I sent 10k to a name close to mine and that was A LOT for me back then.
I waited a month thinking if he wasn’t around it would be returned in the mail but he was still playing and he kept it.
yes and no
I send BOA items with high value mail. Before DF, I used heirlooms.
I haven’t sent mail to the wrong person (if I am sending it to myself).
I haven’t really gotten anything mailed to me that wasn’t mine, and I’m glad about that, lol.
Randomly getting gold in the mail these days, I’ll just return to sender.
like phoenix said send a boa items with any mail
I got mail from the wrong person. At one point, years ago, our landlady decided she was sick of the lot of us, all her tenants. She sent us all evection notices. But she got them all fouled up and no one got their own notice.
She calmed down the next day and nothing more was said about it.
I sent a bunch of gold to the wrong character (not one of mine) once.
I put in a ticket right away and a GM redirected it back to me since it hadn’t been claimed yet.
You know, back when Customer Support was more than some outsourced office in India who are given no power to do anything other than charge your account.
Never gotten mail from anyone else by mistake.
I have gotten the wrong item from a GM, presumably by mistake. They were helping resolve a loot issue (we had masterlooted a ring to a guild member and then realized they had the exact unique-equip ring in their bags already) and sent the highest possible version of the item to me (the person who would have gotten it if we didn’t mess up). So our Normal Horridon kill got me a Heroic Thunderforged ring.
Maybe stuff like that is why GMs don’t have any power anymore…
To, nope. But from, yes. I just clicked to return to sender.
Yup if I send anything major now I attack a pet bandage, or a rep token if I have one handy
best thing is the heirlooms
Every time I almost accidentally send something to the wrong character, I get a popup warning me of such and fix my typo. I’m assuming it’s from the addon Postal.
On two occasions I mailed gold to someone else, though luckily they were decent enough to mail it back 
I pretty much do this now.
I used to send caged battle pets to randoms on my realm…particularly Phoenix Hatchlings, I wonder if this is some form of mail harassment 
Back when auto-fill for some stupid reason would choose the name of someone nearby.
The system warning has saved my sleepy self from sending gold and gear to others accidentally. I once got a mailbox full of metal sent to me by error. I sent it back.
I sent mail to Risqueness that was meant for Risquéness. But both are mine.
I’m sure I’ve done it, although not in recent memory. I normally check my spelling to ensure I’m not losing stuff 
I’ve never received any, that I can recall, although these days I wouldn’t trust it (not that I wouldn’t have returned it anyway, honest mistakes ya’know, but these days I’d be nervous about it even while returning it).
Fortunately, not that i can remember. lol
I’m pretty careful with mail. But I do get annoyed when people have names similar to the toon i want to send to, so they show up in the auto-suggestion. 
It was a long time ago so the memory is fuzzy, but I do believe someone accidentally sent me a bunch of gold and items (I think armor?)
I was so confused and I messaged them back like, “uh, did you get the name wrong or something?” lol
Spoiler: they did.