Err sorry but why should I?

As is the norm in my job, I was presented with a variety of miserable sods complainingtoday, but one had to stand out as one of the most arrogant people I have ever come across.
It comes to her turn after queueing and I greet her in a pleasant manner(of course!) Face like thunder...which has struck Medusa and flown backwards through a bush in fright, she dumps a tin of varnish on the desk, without so much as a word.
I, rather stupidly, presume that she intends to purchase the item, as my powers of mind reading had crashed at this point in the day. As i move to scan it, she pulls it out of the way and demands what I am doing, in a frustrated tone.
Left speechless at having my head ripped off, I say nothing and she offers elaboration in the form of: 'I bought the wrong one, didn't I?'
Oh, of course you did. How stupid of me. I should have known, having watched you get served and followed the continuation of your life right up until the point you realised it was the wrong shade.
By this time, a rather spacious friend who resembled a bulldog not just chewing a wasp, but eating a hive of killer bees with vinegar, had joined her.
I am telling the orignal 'lady' to go and fetch the one she wants so I can do an exchange.
She turns to Rex and says:'Oh so I have to bloody go and get it myself as well as if this wasn't complicated enough.'
Ahem. Sorry, was it me who was dumb enough to come in and choose the wrong tin? Why should it be my job to run after you just because you make mistakes. Like i haven't got a million better things to be doing for better people. And if that is her idea of complexity, presenting a receipt and fetching a tin with the same words as a piece of paper she's holding on the front of it, then she needs to get out more.
Bearing in mind that the varnish aisle is adjacent to the service desk and barely a metre or two away (i could see the tins from where I was standing), I offered to call over a colleague when she stormed off.
By this time she was already up the aisle but Rex says: 'Yes that would be better'
I reasonably said that it doesn't matter now as she was already there. But no, I am once again mistaken.
Rex proceeds to get out her phone and call her friend! I watch, forwning as she says down the end of her phone: 'They're going to get one of their staff to get it'
The woman then emerges from the aisle, bereft of a tin of varnish and arrives back at the desk, offering a hard look. 'Well, where are they then?'
I explain that I haven't yet called anyone and call 'any available assistant' over the tannoy, as if I asked someone specific to help this customer they would have surely presumed I held a grudge.
A member of staff arrives fifteen seconds later and jovially asks how he can help.
'I want a tin of varnish' Helpful.
'Not aproblem. What kind?'
'Ronseal.'
'Okay, that's fine. What colour do you need?'
*looks at Rex* 'Christ sake what is it, twenty f**king questions!? Redwood (or whatever it was!)'
*less jovially than before* Okay I'll see what I can do.

Cue muttering of 'it's not rocket science' between the two women.

By this time a queue had built up and my pathetic excuse of a colleague was too busy talking to someone else and writing a shopping list, so I asked the next person if they wanted help. As this much more pleasant lady began her enquiry, The ROnseal lady butts in saying:'excuse me, you're meant to be serving me'

I offered a curt:'You're being served' but she wasn't having it. But she didn't turn on me. no, she had a go at the customer. 'Wait your turn!'
Before the situation had a chance to escalate, my other colleague returned with the tin and, after checking and double checking and examining with forensics and triple checking that he'd picked up the right one, the transaction completed and they went on their way.

And I hope never to see them again. I won't hold my breath though as the awkward ones are also most common ones.
Such is the world of customer service.