It's quite common to give instructions or warning to do or not do some thing, but no reason why or explanation of the likely consequences.

Description by example,

'''Do no stare into the beam'''.  Why not?  It's likely to permanently damage your eyes; your blink reflex may not protect you fast enough, or you may not experience discomfort before the damage is done.

'''Do no override this method'''.  Is that because the code is being called in some way that means your override won't take effect (easy enough in Perl)?  Do you not trust the later programmer to figure out '''all''' the consequences, yet somehow unwilling to figure them out and write them down?

'''Use only Acme cleaning products to clean your Acme Wiggle-o-tron'''.  This one is harder to figure out.  Is there some cleaning product which will damamge the Wiggle-o-tron?  Is this going to be cosmetic damage or a complete gumming up of the works that breaks the machine?  Or is it because Acme sell some slightly more expensive cleaning products and would be much happier if you didn't use the cheapskate unbranded ones?

'''No entry''' road signs are a little different because of the need for simple instructions in a small space.  There has (presumably) been a traffic order, and it's now a rule of the road that you don't drive the wrong way down this one.  In some cases it's obvious why (sliproad coming off a motorway) but in other cases it's just a choice the road planners made a few years back.  Breaking the rule is likely to cause an "accident", but that's now.  Could it be OldRulesWithForgottenReasons?

'''I'm going to count to ten.  One ... two ...'''.  This works on children who suspect Something Really Bad will happen if you get to ten, but I heard of a mother who asked her parents, "What should I do when I get to ten?  It has been getting very close recently."

Sometimes the reason is given.  '''Do not put any other mark on the paper or your ballot may not be counted'''.

Sometimes there's no obligation to give a reason.  '''Authorised personnel only''' or '''Private - keep off'''.  The "else" may be as provided in law, because the law allows the owner to choose who is invited in.

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I believe this is in most cases an AntiPattern.  My flippant example: opening a tin of espresso coffee.  The lid has warnings about opening it slowly - pull the ring gently to crack the seal open, let the pressure equalise, then peel the lid out.  This is the first ElselessWarning I remember meeting and rejecting, but there are certainly others.  I was rewarded with a small fountain of finely ground coffee. -- MatthewAstley