A bit over twenty years ago I was a young CS student who had volunteered to maintain our "commented directory of lectures" (kommentiertes Vorlesungsverzeichnis) with two other students.
A directory where all the lectures, seminars etc. were listed, with a short abstract what would be covered. This was student-led, but basically everyone used it over the "official" directory, and we got computer resources and some small payment for doing so.
When we took over we inherited a convoluted system of Perl(?) scripts and some database.
We were mailing professors for information about their courses, mailing reminders, and so on. This was all semi-automated. Oh boy!
Professors were really helpful, fast to reply, with high-quality abstracts of their lectures. Except one.
We had decided to include other departments' lectures that were cross-listed in our department, so it was a professor we had never met.
Silence. Absolute silence, we never got a reply.
Until after maybe three mails and a few weeks his secretary sent us a nasty mail: how dare we insult this professor, an absolute authority in his field, with such an inadequate style of address?
We were perplexed. Our mails used the official address "Sehr geehrter Herr Professor X".
(It sounds weird in English, if you translate it literally it's "Most revered Mister Professor X".)
We looked up the mails we had sent them. And were aghast. The mails to this professor (and thankfully only this one) started with "Sehr geehrter X" (double space). Check your database entries for NULL, dummy!
Yes, our mail was unprofessional. We should have handled the database part better.
But this professor was really out of line. Who thinks such an address would be anything else but a mistake and take it so personal as to sick his secretary on us in such a way? "Our" professors would have simply ignored that faux-pas, a few of them would have had a very entertaining riposte, I'm sure, but certainly no bad blood.
How did it end?
We apologized profusely. Never got an reply to that, either. So we simply dropped the lecture from the directory and it wasn't included. We made sure he would never get another mail from us. Nobody ever complained.