I remortgaged around 15 years ago when the interest rate was about 5%. I took a punt and opted for a tracker from Woolwich (now Barclays) which was 0.74 above base rate. The base rate then progessively dropped until it was about 2.5%. At that point Woolwich said that they would have to hold any future payment at 2% plus the 0.74 as they hadn't factored in rates dropping so far, despite there being no "collar" (I think they are called) in the terms and conditions.
Cue myself and many others launching an on-line protest via the Money Saving Expert website saying that Woolwich/Barclays had no right to do this, that it was their fault that they hadn't put a restriction on the tracker, and that if interest rates had risen to 15% as they did in my parents' time, would they allow the customers to impose a collar on the rates to say around 7%. Like fcuk they would.
Legal action was threatened via the website if our mortgage agreements were not honoured and Woolwich/Barclays backed down. Interestingly, as we were posting on the MSE website, WB changed the T&C of all future mortgage agreements to allow for the imposition of a collar, from memory set at 2%, so they were obviously monitoring things closely. Rates continued to drop to 0.1% so I've had the luxury of a very cheap loan thanks to the fcukwit at Woolwich who didn't set a collar in the first place and cost them millions. I wonder where they are now - probably promoted if my experience of large corporations is anything to go by.
If I had taken out the mortgage a few months earlier the rate was set at something like 0.5% below base rate. There was some discussion on the MSE site when the base rate dropped below 0.5% whether WB would now have to pay the mortgage customer money. The consensus was that there was a clause not allowing this during the life of the mortgage, but WB would have to redeem money owed for the negative interest rate at the end of the mortgage.