An opera company I won't name just made an entertaining error in the email announcing their 2017-18 season. And I can tell you exactly what happened, too.
The email announces four operas. One of them is by a composer who is also represented in this year's repertory. The header area of the announcement names Opera A, which will be done next year. The body of the email, with details about each of next year's operas, lists Opera B...which is in the 2016-17 season.
I'm pretty sure that they used last year's season announcement as a template and somehow didn't update all of the text in the body of the email. The composer name was the same, the costumes might be interchangeable (depending). The link in the header goes to the 2017-18 season, the link in the body goes to 2016-17.
The moral of the story is that your mailing list management program contains a feature that allows you to send drafts to yourself and anyone else you think should proofread outgoing email before you send it to the thousands of email addresses on your mailing list. You should use this feature liberally.
I obviously don't know exactly what happened; maybe someone inadvertently skipped that step or maybe someone thought they were sending the email to 10 people rather than the whole mailing list. But I spotted the error about 2 minutes into reading the email, and probably a few other recipients did too.