Hi all, looks like it's been a while since anyone has posted on this but doesn't mean the problem has gone away!
Me and my girlfriend are both using GalaxyS2's, one supplied in France (hers) and mine that came from the UK. She does not update her op sys and still on GB, mine is updated whenever the phone crashes big style as noticed before that op sys update can end up wiping half your installed progs. As such I have tried GB, ICS and now on JB 4.1.2. Both phones still do and have always done the intermittent alarm silencing thing at random with the stock clock prog. Sometimes it can be months of working fine then will suddenly start playing up, mine just did it again this morn. The screen lights up and sometimes starts the alarm for about a second while other times it just silences itself straight away, but in all cases, when I have noticed it happen the screen does wake up. Best bet is to set multiple snoozes so it can have multiple tries, but its not right that this problem still exists after all this time, come on Samsung, though I think they will blame Android and visa versa. As it is we have got used to setting both phones, with multiple snoozes, if there is something important we have to do. If either one goes off we are concious of where and what the state of the other one is so sure its not just a case of phone accidentally being silenced in pocket or missed, it's a real problem so fix it you guys.