Why "was sang" strikes a false note

What kind of error is it?

It is an error in verb conjugation, specifically the misuse of passive voice and tense.

Why do people make this mistake?

People often make this error by confusing verb forms and constructing sentences that attempt to use the passive voice incorrectly. It involves misapplying the past participle "sung" and the past tense "sang" in places where they do not fit grammatically.

What is correct?

"Sang" is the simple past tense of the verb "sing". The correct past participle verb form required for passive constructions is "sung". The passive voice should involve a form of "to be" and the past participle; thus, "was sung" is correct.