My Snow Leopard crashed while I was switching from Itunes to the browser.
Basically everything got stuck and a the current song second started to run in loop.
Is that normal ? Can it happen very rarely ? Or should I worry ?
You should worry if it happens to you periodically. If it happened only once, my guess is one of the background process Mac OS X runs from time to time got stuck for a moment.
BTW, when you say "crash", did you have to reboot your computer? Or did it just got stuck for a period of time?
Anyway, if it crashed, the best you could do is send the crash report to Apple when prompted and check the Usual Suspects:
This is definitely not normal.
I started having this problem after I installed "Lion". It happens often, more than several times a day - it freezes, and the only solution is to push the turn off button, turn off the computer, and turn it on again.
I did many researches on this issue, also went to the "Apple" store's "Genius" bar where they reinstalled my operating system which did not help at all. They also checked my hardware and said that everything is okay.
In my opinion, these might be "Lion's" issues, because lots of people have the same problem, and the problem mainly occurred after the "Lion" install. However, it seems that "Apple" does not even try to fix this issue, or even if they do, they are doing it poorly.
Moreover, you might find this useful:
P.S.: forum to discuss about this issue: https://discussions.apple.com/thread/2768351
I hope this helps.