Since salt and sugar look alike, its not uncommon for people to accidentally swap the two. The substances look alike, and many jars that contain salt only have an 's' on them, which might make people think they are dealing with sugar.
On April Fools' Day, many people (especially children) traditionally perform practical jokes. Some people put a lot of effort into their pranks. Others don't. Most people have, when they were young, put salt instead of sugar into the coffee of their teacher or parent. In theory this could be funny but in reality there's no real comical element to the whole thing. Coffee with salt in it just tastes really bad.
Another popular salt-instead-of-sugar-prank, is putting salt in the sugar bowl. Again, it might seem funny, but chances are slim you'll be laughing when you're sitting at the table, surrounded by people with ruined food.
LittleHawk told me about that last prank