strangest bug you found code example

Example: what is the strangest bug you found

What is the strangest bug you found?
I remember once I found very strange bug and it was like this.
I was testing Email functionality. 
We have internal email client inside the application and 
I was sending email with attachements.
For some reason certain type of attachements 
couldn't open when email was send in the morning.
Very strange! Really strange issue. So in afternoon
everything was fine but if you open the
attachment it in the morning it was
giving 401 unauthorized error. 
So you are trying click on excel file that was
send over the email as an attachment but instead
opening file it was giving an error. Very strange.
So what developers did they run the application
locally we look into that together and they just
connect it to the test enviroment database to the
same enviroment So they could see same data 
same user everything. And also we were looking
into this server where documents stored.
So what actually happens when you send attachment or 
any kind of file what application does it stores
that file in the server aswell.
but the adress of the file will be encrypted.
but in the server we were
able to see those files nothing has changed but
when receipent of the email
trys to open it his getting error. 
We were looking into server logs to
investigate this error and couldn't understand
what happened. So it wasn't 
the bug yet because nobody could understand
where is the problem,
it's a kind of problem but to determine
we need to find out
where is the root cause  of this problem. 
To determine it's a bug indeed. 
And solution was even stranger we just
re-deployed application
that is responsible for this files hosting. 
So it was seperate that is
 managing access to files. And simple re-deployment
 solved this issue.
Developers spent whole day on trying to figure out
what was the problem
and investigate everything after re-deployment there 
was even no code change 
everything was the same but refreshing solve the problem.
It was the strangest issue I have ever seen