So if you walk around presenting yourself as Java instructor and it turns out you don't even know C, you'll be the laughing stock of everyone with just a little insight into the field of computing.
In my view, this might be an old-day passion. If the instructor knows what he/she is going to teach and if he/she has some good experiences on that particular field, (like
Java for a Java instructor), I don't think that person is going to be facing any trouble while instructing that subject. Anyone would not ask "how to do this in C" while someone is instructing the subject of Java.