Here's one that might get you scratching your heads - it certainly has me!
I'm using a form to submit content to WordPress' 'update_post_meta' function.
The form itself is pretty simple, just a range of input and textarea fields. On submit the action directs to the same page where some PHP takes over to do validation on the $_POST info before submitting to Wordpress. Now it all works perfectly, except one very strange bug:
Whenever you use the word 'casino' in any of the fields, instead of submitting the form, the page refreshes and throws up a 404 error, even though the address bar shows a perfectly valid URL! I don't believe it's my PHP code as I don't think we get that far in the process, so I'm thinking it's a server thing, but I can't imagine why. This happens on any form across the whole site and I've also got the same result on other sites hosted with the same hosting company (Namecheap, FYI).
So, the question is, can anyone suggest a reason why sending that word in particular to $_POST would break a form and cause that error?
PS. I'm currently solving this using a replace() javascript function so 'casino' never gets sent with the form, but if the 'casion' issue can solved I would be very grateful. It would at least stop me puzzling over it.