what is the strangest bug code example

Example: strangest bug 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