A customer asked to pay in cash ("take cash?"), shuffled notes in his wallet, handed over a £10 note to cover the bill of just over £8. Before I could place the tenner securely in the till, he mentions about having a £20 note ("got twenty"). 
I obviously respond that the £10 note is adequate, but the customer got visually and audibly distressed that he wanted to pay with the twenty not the ten. Expecting the twenty to perhaps be a fake bank note, I doubled checked it and found it to be genuine. So I returned the tenner, and gave him the change for the twenty given.

At absolutely no moment in time did he mention he needed the tenner - perhaps he was going to purchase something elsewhere for a few quid and didn't want (potentially) a lot of change - which would have cleared it up and make it a whole lot less awkward. Except he already had a few £10 notes. Maybe he collects them. Who knows?!

The interaction just wasn't friendly and he spoke with few words. From the attitude, I got the impression he was someone saving his breath as a sign of disrespect, rather than having a disability or learning difficulty. But I think what I could change next time is just let the customer choose to change the bank note given.
Fifteen minutes later, he is having a much fluent conversation with (what I presume was) a friend. Really strange.