(Humorous answers, please.) What’s the worst part of sitting on the fence? ~

It can really hurt when you fall.

You might poke 
yourself where it hurts,
or worse yet get a splinter
in your butt and have to 
have it surgically removed
because the fence was painted 
with lead paint and they
might have to remove your Butt.

Splinters. 

There are just some places on the human body where it is virtually impossible to take a splinter out of by yourself.  So off you go to the emergency room.  Getting past the screening nurse is bad enough (you have a splinter where?), but then there's the ER doctor (usually female) who lays you face down, props up butt into the air, and shaves the area around the splinter.  Then, while trying hard not to laugh, she begins gouging out the splinter (and she’s none too gentle about it).  Now, depending on where the splinter is lodged itself (fleshy part of butt/thigh, taint, or worse… your (…um…) "sensitive” parts), this can be either quite humiliating or excruciatingly painful - especially if it’s a deep splinter.  You see, they don’t anesthetize the “area” before excavating the splinter; they just spray it with what feels like liquid nitrogen (extremely cold), say, “this might hurt a bit”, and then start trenching with the biggest scalpel in the world.

There you are, lying in a multi-patient room in the ER, baring your “all” to a complete stranger, 3 other patients and their “visitors”, all while trying hard to act manly and not scream, yet praying you don’t wind up castrated… all because you sat on a dumb fence.  And if that weren’t bad enough, suddenly you start feeling gaseous.

Don't sit on a picket fence and never whiz on an electric fence.

The pickets poking into your buttocks.

For the fence around my house, it's obvious.

Trying to decide which side where the grass is greener.