People hate when they are stopped in midst of something. If they have joined a queue and guy at the counter tells them that this is wrong queue they are rightly enraged. If people are buying something online and on 3rd step if you add some caveat or clause that will change their buying pattern or decision to buy then they will not feel happy. By stopping people in midst of something you are wasting their time and their effort so they will definitely give you a piece of their mind.
Give people seamless experience. Smoother the experience, more they will appreciate you to value their time and effort.
If you want to tell them some instruction or note, tell them before they embark on their journey. They will accept your note very gracefully and might not even budge. The same instruction\note given in between their journey will force them to leave you for someone better.
Remove all the hurdles from every journey.