I spent a few years working in the banking world (no, not investment banking: the super slow-moving savings & loan stuff. If you think that doesn't sound like me, well, it doesn't sound like me :)).
While I was there, the international banking crash of 2008 happened (not my fault). Around that time, a couple people mentioned the idea of negative rates to me, and that was shot down by anyone with "real experience" as being totally mathematically impossible; they treated zero interest like an asymptote, not a threshold. I kind of suspected they were incorrect though I wasn't sure I understood the economic theory well enough. Well, I guess they were incorrect: https://news.yahoo.com/swiss-alternative-bank-breaks-negative-rates-taboo-055303880.html
I find it amusing how hard people have to work to rationalize negative interest rates. It isn't that complicated; if my savings account rate drops from 1% to 0%, or from 0.25% to -0.75%, the absolute value difference in my account balance next year is the same. The rest just follows from there.