People hate when they are stopped in the middle of something. If they have joined a queue and the guy at the counter tells them that this is the wrong queue they are rightly enraged (The counter should have been clearly labeled and should have been very visible). 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 the middle of something you are wasting their time and their effort so they will definitely give you a piece of their mind.
Give people a seamless experience. The smoother the experience, the more they will appreciate you to value their time and effort.
If you want to tell them some instructions, tell them before they embark on their journey. They will accept your instructions very gracefully and might not even budge. The same instruction given in between their journey will force them to leave you for someone better.
Remove all the hurdles from every journey.
PS: This post was originally posted on hoodasaurabh.blogspot.com