BT - this still irritates me 10 years on. Decided I was moving house so I rang to see how long I had left on my phone and broadband contracts as it was coming up to a year since I moved in. I was told they didn't need notice, I would just phone them on the day I was leaving and cancel and I'd be out of contract by then. Great, that was easy.
Except no, the incompetent fool cut me off there and then. No phone, no internet, and the arseholes gave my phone number away. I was charged early cancellation for both contracts, the 'final bill' was £500 and I couldn't even figure out how that was calculated since my contract only had 3 weeks left on it.
I spend so long on the phone to them, constantly on hold, being passed backwards and forwards, eventually being allowed to complain, they believed me, they didn't believe me, they had recorded the original call, the hadn't recorded the original call. It took months, meanwhile I moved house.
Then I realised that they'd taken my 'final bill' amount twice. £1000. Furious wasn't the word. I ended up back on the phone (repeat the previous paragraph).
Eventually they 'realised there had been an error' and refunded my money: this was out of the blue, not related to any recent phone call. I was almost demented at the end.
The next is Garden Buildings Direct who I ordered a playhouse with, paid in full and was given a delivery date. 3 delivery dates came and went, my status on their system went from 'waiting for dispatch' to 'on hold'. When I tried to contact them, I couldn't. They were only contactable through leaving a message or their online booking system. I used both for weeks, still no response and still no playhouse.
I DM'd them through Facebook with all the information, their automated message requested the information I'd just sent them. I eventually got a refund, initially the assistant who phoned me back argued as to why I wasn't willing to wait until my item came back into stock - I had never been told it was out of stock! So a whole summer where the house could have been used was wasted. We aren't going to bother again.
Why these companies have to be so incompetent I don't know, surely they survive on recommendations and so stupid mess ups shouldn't happen in the first place.