Oil Filter Cap

Has anyone known of it undoing itself and leaking?

I've now had 3 instances of it happen to me, the first time the gasket looked like the oil had forced it out in about a 1cm place, the second time I couldn't believe it had happened again, and the third time I was going down the road cold engine and I got what I thought was the old oil pump packing up thing, when I looked under the hood the whole cap was out and sitting on top, it hasn't done it since and I've always wiped the threads clean and used new seals and nipped it up tight but not overtight, the second time it leaked I had marked it incase someone was able to access the car and was loosening it off for whatever reason, the cap although loose had not moved off the mark, still alighned, the only suspect I can find is the cap thread has about 1cm missing halfway down the thread, would this be enough to cause this to happen, I'm loath to spend £26 on a plactic cap right now.

If anything, they are often a pig to undo. Stick a bit of gaffa tape on it. :D