I agree with the answers pointing out that this may be discouraging people that notice, and that you really need to see if it is having th effect that you want by looking at the other resumes, but...
This is a weird request, and it is imprecise. It doesn’t say to include a paragraph of the single word Sunshine, it doesn’t say to give your opinion of the book by Robin McKinley. It just says include it, but include it how and why? Perhaps as the first letter of every other sentence? Last letter of every sentence? Prime character in each of the first/last 8 sentences?
It has the exact same problem of dealing with imprecise coding requirements and throwing out those that don’t ask for clarification — it’s a pass/fail, where the only way to know the right answer is to read the askers mind!
Because this just as easily could be a bad edit by whoever posted the ad, in which case including it may be helpful or terminal — what kind of idiot would fail to realize this was a mistake and then actually do it? How many noticed but decided not to do it?
The David Lee Roth example is much clearer, in one section of the contract it requires M&M, in another it specifies that no brown M&Ms are allowed.
Say that the letter is supposed to be for the attention of Sunshine. Say that you give extra points for Sunshine being included. *Include the value as Base64 encoded and say to include the result in the cover letter. Give a reason and a context, make it make sense, irrational requirements are bad. If it makes sense, then filter and see if is doing what you want. As it is, you are just as likely to have people not do it as do it even if they notice and want the job.
*Stole this idea from Neil Slater.