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Well this is happening from time to time and it is very infuriating.
The first thing is that safari forgets the passwords on forums and everywhere basically that needs a username and password. It might be ok for a month or 2 but after that I might have to reenter the codes. Does that have to do with safari or with the actual cookies expiring?
And the other thing which I'm sure has to do with safari is the auto completion of the addresses. Usually I just have to enter a few letters and the closest matches will pop up. From time time all these "lists" get deleted. My router's ip address is one of those instances. I don't use it that often in order for me to add it as a bookmark but often enough for the browser to have it stored. Today it couldn't even remember the address. The same goes for other sites as well. Can anyone explain this behavior? Why exactly is that happening?
Thanks in advance for your answers.

Cookies issue could indeed be because they are expiring; the second issue could be because those pages are no longer in your history (which is also time limited - I think Safari is set to remember pages that have been visited for one month by default).

However, there could be other problems, such as your cookies and history getting trashed inadvertently or as a bug. Also, if you Reset Safari... at all?