These are two different ways:
1. Your boundaries can get crossed when you find yourself in a situation when someone (or you yourself) is forcing an experience upon you that you don’t want to have.
E.g. your parents force you to eat something you don’t want to eat. Someone sends you an unrequested dickpic. Someone touches you without consent.
2. Another way in which a boundary can get crossed is when you are in a situation that’s uncomfortable, maybe only slightly, but then lasts too long.
E g. being put on hold on the phone for a long time. Someone repeatedly asking something you don’t really like. Someone promising you something repeatedly but not actually delivering.
This second version may be less visible and harder to identify, especially in closer relationships.
P.s. this post is really how I realize I have a boundary. Not about what I do with them as a follow-up action.
[Photo ID: Wilrieke sitting in bed, naked and covered by a red duvet. The tattoos of beech leaves and a tapir on their left arm and shoulder are visible, as well as a string necklace with a circle-shaped pendant with a peach on it. Wilrieke looks like she just woke up – which they did.]