“Depressed Storm Drains” on a highway I use occasionally. I always wonder what I could do to cheer them up.
Not really hilarious, but brings a smile. In my former home town of Anchorage there were a lot of signs in various parking lots and businesses that stated “No Snow Against Fence”. It’s supposed to let the plowers know not to pile snow against the fence of course, but it sounds like a command from God. And how, in a land where it snows 6 months out of the year, are you going to enforce this rule?
And this isn’t really quiiiiite along the lines of what the OP was looking for, as I purposely made it silly but here goes…
On one of my trips back from a job, I was asked by the ticket agent if my bags had been under my control the whole time. This was during a job with massive hours per day and I was tired and silly. I glance askance at one of the bags and said “well, for the most part, but THIS one has been acting up a bit”. He must have been having a bad day, because it cracked him up. 
where’s the “I’m dee deeDEEEE” smilie? Okay, what’s a C#?
Another one.
There’s a street in a nearby town by town with a dead end sign. It leads to a cemetery.
Signs on Chinese restaurants in Canada that say “Chinese and Canadian Food”.
It’s a computer programming language. It looks like you’d pronounce it C-sharp even in this context, as far as I can tell.
Some of the people at my company are psychometricians. We generally find their actions baffling, so it’s amusing that their position code for the timesheet program is simply PSYCHO.
And there was a big fuss about this amongst some because the symbol for sharp is actually ♯, not #. IIRC, Microsoft released a press note saying about it, saying they used the # because it was easier to type.
The Android smart phone OS knows whether you’re a monkey:
It comes from a way to stress-test programs by feeding them pseudo-random input, notionally similar to a simian randomly jabbing at stuff. You don’t want the monkey to accidentally do something destructive, like erase data, so you check to see whether the user is a monkey and then disable anything that could be harmful.
I love the signs on worksites around here that say “hardhats must be worn on this site”. They make me want to call out “Hey! You can’t go in there, your hat looks brand new!”
Similarly “No Entry, Authorised Personnel Only”; I’m not authorised, so logically …
“Stop Ahead” signs. I always think, “Well, I could take away his bong…”
Or Oregon State’s. A couple of weeks ago, South Carolina played Troy, while Oregon State played Southern California. (Troy apparently has the same nickname as USC.) Made for some fun snickers among the sports fans at work.