Our biology teacher told me I shouldn't go into forensics because I'd have to taste stomach acid to tell what killed a person. Turned out that this used to be a method of doing toxicology but is way outdated and very much against health and safety.
On a less gross note, My chemistry teacher told there was no hope of me getting into my first choice, three years later I graduated from that exact university. Don't know if it counts as a fib, but it certainly didn't ring true.