Never actually had this directed at me, but I know too many HS/college students hear "never, ever, EVER get a credit card!" instead of "never, ever, EVER carry a balance on a credit card." I respectfully disagree with Dave Ramsey in that the credit score game is here to stay, so you might as well learn (and teach your children) to win it.
Also not directed at me but I see/hear it: "You should move out of your parents' house because you're [age]." With the optional addendum "I've been on my own since [age] and it made me who I am today!" Plenty of families around the world make multi-generational homes work in a respectful way. Do what's right for your situation--financially and emotionally.
This one I think I did hear but disregarded: "Go to the best/most prestigious school you can get into." While I think it can totally make sense to bend over backward to attend a truly elite institution (e.g. Ivy, MIT), particularly when the price premium isn't extremely large (say, $5k/year, maybe $10k, but not $50k), I do believe below that point the rankings break down in utility and the education and name recognition you'd get from a given institution is mostly the same as the next one on the list. Once you get below that super-prestige level, it's time to consider value. On the flip side of this, some of the best advice I got was to not evaluate the cost of attending a particular school until after fin aid offers came back. The state school was NOT the cheapest for me (I got a merit scholarship elsewhere) so it saddens me when parents seem to be discouraging their kids from even *applying* elsewhere; maybe it's not common but you could be missing out on a good deal.