That’s another really great question, Zari! Hope you don’t mind if I jump in on this one.
Not all witches cast circles- and the type and function of the circle can vary fairly significantly from witch to witch and practice to practice. If you do choose to use a circle and you or someone else enter/exit your circle without opening a door in the circle or removing the circle, I believe there is the possibility for problems to occur.
The Circle Casting FAQ in the Spells8 Circle Casting For All Guide says the following about this:
Can I leave the circle?
This depends entirely on the practitioner or coven. According to Raymond Buckland, “At no time during the working of magick should the Circle be broken. At other times it is possible to leave the Circle and return, though this should always be done with care, and no more than absolutely necessary.”
I think it’s safe to safe that all witches who do choose to use circles in their practice will agree that, after the circle has been conjured and used, it must then be taken down (click here for How to Take Down a Circle).
That being said, forgetting to remove a circle does not necessarily mean the spell will fail or there will be negative consequences. Here is what the circle guide says if someone forgets to take the circle down:
What if you forget to take a circle down?
If you forget to take a circle down, it isn’t the end of the world. But if you took the time to create a formal energy barrier to protect you, it’s good form to release it and not forget. Over time the circle will dissipate on its own.
Any potential side effects of accidentally breaking the circle will likely depend on the type of spellwork you are casting. If someone is casting protective magick or a healing spell, I think it is quite unlikely that the good energies contained within the circle would cause any unintended harm.
If the circle where a caster was containing Baneful/Black Magick was broken, however, I feel that that kind of strong and hateful negative energy could very well have a negative effect if released onto the wrong target.
For anyone weighing the pros and cons of using circles in their practice, Francisco gave some really great advice that I’ve copied here:
Hope that helped to give you a better idea and answer your question! Wishing you blessed casting- Love and Light! 