What is your worst experience with something getting lost in the mail?

My negative experiences have been along the lines if important papers getting lost. These had to do with deeds and financial stuff with a house I sold 20+ years ago. The papers were ostensibly sent, but I never received them.

Fortunately, the office that sent them had copies (that they could certify as true copies of the original) and the second time, they arrived.

Since then, anything of seriousness goes Fedex where it can be tracked (and trusted to arrive).

I can see screwups with snail mail, it happens. But with a fax? I sent some info to IRS pertaining to an audit they did on us, but when we called to check they said they never got it.
We had to fax the same papers back to them. WTF?

I keep getting mail from people who lived in my appartment. My mail sometimes goes to next door. Government info goes to next door 50% of the time

I sent some papers to my mortgage company but they claimed they lost them. I will be sending stuff to them certified from now on.