What is the oldest surviving bug in Windows?

I'm trying to figure out the oldest bug in Windows that still exists today, but the information I've found is mostly about old vulnerabilities that were eventually patched. For example, there was a security flaw that survived from Windows 95 until 2014, but Microsoft fixed it . Another one dated back to 1993 and was patched in 2010 . What I really want to know is: which bug is still present in modern Windows, never fixed, and can trace its origins back the furthest?