The HSE have recently issued a safety alert about certain aluminium cylinders being subject to sustained load cracking (read: exploding under pressure):
http://www.hse.gov.uk/safetybulletins/aluminium-cylinders.htm
The bulletin goes into how to identify the cylinders which are affected. A good rule of thumb however is any cylinder manufactured between 1963-1995 should be checked by an IDEST centre with visual eddy testing equipment.
If you are in possession of a cylinder which is made from the HE30/AA6082 or AA6351 alloys you should remove from service, release any contained gas in a controlled manner and quarantine it until it can be inspected.
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