Is "misspecification" a correct/real word?
If by "real word" you mean in a dictionary, then yes, it is a word.
misspecification
n. An incorrect specification Wiktionary
Regarding whether people will understand these little-mentioned words, a general rule of thumb is that common prefixes e.g. un-, de- mis- attached to nouns (within reason) will be understood by most people. Even if it isn't an official word, if it sounds plausible, it's probably OK.