If the anaesthetised patient has been cruising along with a blood-pressure of 120/80 for the last hour and monitor A suddenly shows 60/20, while monitor B continues to show 120/80 then monitor A is likely to be wrong.
This actually happened to me (sort off), anaesthetised patient
(me) suddenly showed 0/0 which was correct & needed immediate intervention. Suppose monitor A had got stuck
at that moment. My training in electronics is a bit out of date,
but in my day "stuck at" was a problem which needed a bit of
attention.