Tosfos says in Shabbos Daf 3a that even when there isn't a problem of lifnei iver since the person would be able to get to the prohibited thing in another way, still there is an "issur d'rabbanan" ...
Why does the Pasuk (Deut. 4:25) switch between singular and plural when building up to the consequences of what "you will [do]"? "When you (sing.) will have children and grandchildren, and you (pl.) ...