A word for an intentional error or absurdity inserted to check whether audience read an entire passage

I have a tendency to write long(ish) and detailed emails at work that I suspect the target audience don't bother to read sometimes, so occasionally I will insert something obviously silly just to see if anyone picks it up. Is there a word for that?

I am also aware of the similar practice of inserting intentional errors in, for example, maps for the purpose of detecting copyright violations - I imagine that would share the applicable terminology?

From Wikipedia...

fictitious entry
Fictitious or fake entries are deliberately incorrect entries in reference works such as dictionaries, encyclopedias, maps, and directories. There are more specific terms for particular kinds of fictitious entry, such as Mountweazel, trap street, paper street, paper town, phantom settlement, phantom island, ghost word and nihilartikel.

From Dictionary.com...

mountweazel
Any invented word or name inserted in a reference work by a publisher for the purpose of detecting plagiarism.
From Lillian Virginia Mountweazel, fabricated for the 1975 New Columbia Encyclopedia.

From Wiktionary...

nihilartikel
A deliberately fictitious entry in an encyclopedia or academic work, generally identifiable as false, usually included to brand the intellectual property so copies can be identified.
Considered a loan word from German; Latin nihil- nothing and German artikel article; from a fictitious March 2004 English-language Wikipedia article, referencing a September 2003 article in the German-language Wikipedia now titled Fingierter Lexikonartikel.

The terms trap street, paper street, paper town, phantom settlement, phantom island are all relatively transparent references to fictitious items on maps/atlases/etc.

As you can see, Mountweazel and nihilartikel are relatively recent coinages. Personally, I call fictitious entries in dictionaries ghost words. My favourite example being contrafibularities. I'd been laughing at that Blackadder skit for years before I realised it was actually a "mock Latin" pun on leg-pulling (fibula = a bone in the lower leg).

In the United States, such a nonsensical insertion might be referred to as a "brown M&M" clause. As this article from Snopes.com reports, the rock band Van Halen included a contract provision at performances that the band be supplied with a bowl of M&M candies (which normally come in a mixture of colors—red, blue, yellow, green, dark brown, and (formerly) tan—backstage, with all of the brown-coated individual candies removed.

The point of the red-herring provision, according to Snopes, was simply to serve as an easily verifiable test of whether the promoter/host at each venue had carefully examined its obligations under the contract and met them:

The legendary "no brown M&Ms" contract clause was indeed real, but the purported motivation for it [to provide legal cover—owing to the promoter's being in breach of contract—for destructive backstage behavior that the band would otherwise be liable for] was not. The M&Ms provision was included in Van Halen's contracts not as an act of caprice, but because it served a practical purpose: to provide a simple way of determining whether the technical specifications of the contract had been thoroughly read and complied with.