Definitely not the same level of WTF, but I worked on one of my favorite bugs I've ever seen super early on in my career.

I joined a tiny digital agency maintaining wordpress sites and about a month in one of our customers files a ticket that their website was broken. Just a white page, no error, no nothing. I ask my boss if I should switch to work on it and he goes, "Nah, this customer does that every few months and there's never anything wrong. It's something with their hosting or something (they were self hosting a site we built and maintained). Just take a look at their site when you have time to say you did and close it as can't reproduce."

Two days later I have some spare time so I take a look and sure enough, everything is working as expected. A few months pass, same ticket from the same customer, I pick it up a day or two later and everything is working as expected. A few months later, same thing, but this time I have nothing I'm working on so I pick the ticket up immediately and sure enough the website is broken. I immediately show my boss and he's like "well, I'll be damned" and then tells me to fix it. I poke around for a few hours, but can't figure it out, so I call it a day. When I get in the next morning, things are working as expected, so we're both like "wtf?"

I don't remember how exactly I ended up figuring this out or even all the details of the bug, but the root of the issue ended up being that who ever wrote the code for the site had some code that revolved around the current date and they'd hardcoded that there was always 30 days in the month. When ever the current date was the 31st of the month this code broke and took down the website, but by the time the 1st rolled around the code worked again.