Actually not true, more then once when I traded with people, the other pressed ok by mistake without waiting for me to put in the item. I traded him it to him immediately after and reminded him to pay close attention in future trades.
Trade scamming is a very simple scam which can easily be avoided by taking one's time, but purposely attempting to rob someone of their items/noahs is still a scam no matter how simple it is. Or whether or not you think they should have checked closer first. If the intent to trick the other is there, it is a scam.
Remember also that some fairly young children play this game too, the person you "get over on because they were too foolish to check" may in fact be a 7-10 year old kid who is not as likely to pay close attention. Though the fact that some people in this game may enjoy getting over on 9 year old kids is also true..