Well,

it does not make much sense.
As I already wrote above, error 500 does mostly does not relate to web site "data" - that is what HTML code was generated or whatever is stored in DB. So neither any URL stored in DB should be "real" reason of that.

Try some more server-level debugging:
Does the server restart help in such case? (try not to touch URLs or Joomla, just try to restart webserver)
Are you using some eccelerator code on your server? eaccelerator for example?

So ur saying restart apache? or the whole computer?
I know it seems strange but thats exactly whats happening.
As for the accelerator , im not really sure. I didnt install anything like that, maybe its something default.
I have centOS.
So i gues i will try to restart apache next time i have this problem.
But if it would help, im not sure how much help it would be.
Anyway.
I will come back with the answer to this (restarting the apache service or the whole pc).

Well,
if it would be help, then I would be pretty sure you are using some kind of acceleration and cleaning its cache solves the problem.
We had similar problem when using eaccellerator. It seems a bit buggy. Also other users were reporting similar problems. The result was usualy not 500 error, but blank page or other kind of error, but that were not really making sense.
So, if the behaviour seems unpredictable, then I would guess for error of this kind. If you will browse older posts a bit, you will find the discussions on this topic.

The site just got the problem.
Restarting the service didnt resolve it.
All i could do is purge all the urls.
I dont know what to do.
I saved the sql of all the sef urls.
Can this help ?

So what can i do ?!
Can anyone help !?

Well, another think to try would be disabling JoomSEF cache.
Or, if it gets the problem again, then do not purge URLs, but just clear JoomSEF cache.
There is a possibility, that the file locks do not work as supposed on your system and the cache file get corrupted. This can be then solved by changing the file locking option in JoomSEF configuration or disabling the cache.

However, usually, this problem does not generate 500 error but some other one, commonly a parsing error.

Except this, I have no more ideas. It might be also some kind server specific problem that appears only in combination with JoomSEF and URL rewriting. E.g. the URL generated is conflicting with something on the server and similar.