Period by consensus
Generally, only items considered period are really accepted at SCA events. A few (really useful) items are considered by some to be in period by consensus due to the fact that their omission would make life much more difficult and less fun. Items like this may include:
and many others.
In Ealdormere, several items, including chocolate and coffee have been declared "legally period", a formal declaration that they are period by consensus. In numerous kingdoms over the years, Royal Whims have declared numerous items to be in period - chocolate is a common offender. It is worth noting that everyone understands that this is a joke, not an attempt (like King Canute's) to override physics or any sort of attempt at actual scholarship or re-creation.
Many (most?) places simply acknowledge that the items are out of period but use them anyway. In fact, marking them as "period by consensus" simply highlights their OOP nature and makes it harder to ignore them.
The term "period by consensus" itself would seem to be a limted regional usage.
This idea is less accepted in re-enactment groups. Most discourage such behaviour. During displays such activities are not permitted, and during events where they are not on display, only cigarettes make an appearance and then usually out of site. This is considered a minor inconvenience to a few and enhances the event greatly for many.