I've heard a rumor regarding German traffic law that seems too ridiculous too be true, but sometimes the truth is stranger than fiction.

Native Germans (or German lawyers): It has been stated to me that a person can be fined if another person claims they insulted them on the road (middle finger, finger pointing at your head, etc.). The claim goes on to say that the polizei will automatically take their word and there is no defense for the accused. (The logic behind this is that no one will write down your plate number and call the police if it didn't actually happen).

Is there any truth to this? Can someone actually make such a claim and have a fine imposed based solely on one person's testimony?