Criminal trespass is your continued presence after you have been given notice through posting, fences, or an order of an "authorized" person. Willful ignorance doesn't count. Criminal trespassers are subject to arrest and punishment. OTOH, if it would be dangerous for you to proceed because of something unforeseeable (like a freak snowstorm) you would have the excuse of necessity. This would be generally true in common law countries like US, Canada, etc etc.

If you camp on private land you take your chances. But although I have never stealth camped, I would certainly do so if the circumstances at the time - despite planning otherwise - dictate that I had no other alternative. But bear in mind, the irate land owner is probably not a lawyer and may feel entitled to do whatever he wants with you - legal or not.