I'm wondering can the re-occurring of
File Not Found (404) error eventually lead to
Internal server error (500)?
I would definitely argue it is possible if your 404 page was built using a dynamic language like PHP, ASP, etc. (anything beyond what is considered just markup language). The introduction of a dynamic language introduces a wealth of ways in which a page can fail (threading problems, memory management problems, resource locking problems, etc.)...some of which become evident one a single page load and others that are only evident under tremendous stress.