'Off-menu' secret items in restaurants - Either put the damned stuff on the menu for everyone or don't offer it at all - especially when it's an open secret - e.g Joe Allen's burgers.
Sometimes the reason is because it isn't as popular anymore, and so they've taken it off the menu--and so now, only a few of the older cooks know the recipe and it's basically a crapshoot whether you get someone who still remembers how to make it by the restaurant's recipe or not.
For example, there's a place that used to sell blackened chicken with penne. It didn't go over well, so they changed it to blackened salmon with penne. However, Mom was still able to order it with chicken for over a year after the menu change. However, most people don't want the salmon, they want the chicken. It's less of a "why the fuck are they offering this?" and more of a "we're trying to phase this out without confusing new customers."
On topic, go away for 5 minutes and the whole bloody forum changes.
Um...the changeover happened on Wednesday. It is now Saturday.
Yeah, 5 minutes in internet relative time.
More like 5 years.