ok,
having that gap makes the map vis and compile much more complicated than it needs to be. For some reason it appears like you have some sort of room down there which means it doesn't actually constitute a leak, but it will render and include it in the VIS when you come to compile it. for the sake of being a good mapper and good practice, and for moving one line 4 pixels to the right, you should do it. Don't ever leave mistakes like that because you can't be bothered to fix them even though they are so simple. Yeah it's no masterpiece, but if you get in the habit of shrugging it off, your later maps are likely to be riddled with careless mistakes too.
if you want to be able to stand on the fence, but also shoot through it, copy the fence brush, paste another copy in, move it one or two units above the original brush and make it a clip brush (apply the clip texture to it). It will be invisible then, players will stand on it, yet paintballs will go straight through it.
- Dagless