US, Israel, Hong Kong, Japan, quite a few places it can happen.
There are several ways it can happen, tho the second is only common in the US:
- The terminal can not support or is not enabled for EMV, in which case magnetic stripe uses signature in most countries (online PIN in some).
- The terminal can support EMV, but not offline PIN (only really common in the US market).
- Offline PIN can be locked out (PIN try counter tripped) from three wrong PIN attempts. Signature is above online PIN on the CVM list for the Monzo card, so the card will revert to signature (which Monzo may decline, haven’t tried it).
- Contactless can be used for more than the local No CVM limit (e.g. £30 in the UK) and online PIN not be supported by the merchant. Monzo is likely to decline, but the transaction could be approved conditional on a signature, as in 3. (and yes, there are banks that will approve at least low-value transactions in 3).
- PIN could be bypassed by pressing ‘enter’ without entering a PIN or similar, depending on the terminal. This will then revert to signature, again. It may or may not be declined.