On pax PA, I just used to start with "Hello everyone...."
But an Andover captain once told me of the time he was flying some soldiers from somewhere to somewhere else and decided to address them over the PA. Being a bit of a joker he told the crew "Right, I'll wake those slovenly buggers up now and get them to shut up and listen..." and launched into his PA spiel.
Which would have been fine, except that he switched to PA whilst joking to the crew, so the soldiers heard everything. According to the ALM, the effect was rather startling!
PA switch pigs on the VC10 weren't unknown. OC10 Sqn once amused everyone else on the Brize Tower frequency when giving a lengthy PA spiel to the pax, but on UHF. He told us "As I released the switch I heard an ominous 'click' and thought 'Oh $hit, that'll cost me". It didn't take long before a very polite, gentlemanly comment from the captain of another 10 told him what a very nice pax brief he'd given, but what a shame that they wouldn't have heard it!