My family experienced that on a micro-scale.

In Poland, banks used to give mortgages in Swiss francs. Since that currency was very stable, everyone thought it was a great deal because you paid a smaller interest rate than in our national currency.

My parents - just like millions of other people - took a credit in Swiss francs.

But then, 2008 happened, and the Swiss franc went +50%, their interest rates went up, and so did the amounts people had to repay. It was - and for some people still is after 16 years - a bloodbath.