For example 3DSecure mechanism redirects customer to another site where user has to put a number received in a text message. My phone is in another room upstairs. I cancel the payment and tell myself I'll do it later. Of course I might do it, but I might forget about it or change my mind.
Another example: (in Europe) from time to time, when making a transfer/payment, I have to generate an OTP using a physical device that I received from my bank. I find it extremely irritating, and I don't take this device with me everywhere, so if I'm making a payment and I'm expected to use this device, I cancel the payment.
I've worked in a place where we introduced 3DSecure mechanism for our payments, and in certain countries it dropped conversion rate while in others (where people are used to such mechanisms) it remained the same