When people want to be difficult - usually because they have made an error or changed their minds, they can be very sneaky. My business (now sold) was in financial services. We did not give advice at all, but we provided access to markets (mainly institutional but some retail). It was routine and common for people to claim that we had made a mistake of price, quantity, timing, buy v sell, wrong stock, etc etc. Almost invariably nonsense.
It's regulated anyway, but we never did anything without it being voice recorded (recordings kept indefinitely), in writing, confirmed, followed up with contract note, governed contractually and so on. Yet some people would still claim black was white. Hearing their own voice played back to them on an email of a recording was usually enough to stop further argument. Anyone who thinks people behave honourably nowadays is more generous than me.