I'm starting to use Illustrator a lot and I have realized the importance of using a good technique when drawing my works of art onto the art board. One of the most important rules when using Illustrator is to close a path when you have finished using it. There are many reasons why this should be done, which I will explain below.
Although when you fill a path the fill takes the shape as you may want, as shown below. It does begin to look messy. As well as this Illustrator has to guess what you want the shape to look like. This usually involves drawing an imaginary straight line. This is better explained by looking at the image below.
This also begins to pose problems when you begin to use some of the transformation tools. Since illustrator has to guess a lot many of the tools don't work. For example the Warp, Twirl, Puker, Bloat etc. The problem with this is that you may want to apply a transformation over a couple of objects. If one doesn't have all the paths closed, this will obviously not work.
Another reason to close paths is for external projects. By external projects I mean selling your work, giving it to others. Many people will want a piece of work to be complete and free of any errors. For example my favourite image stock site, iStock Photo wants all paths to be closed. This of course is for the above reasons.
Solving This Problem.
There are many ways to solve this little problem. The obvious way is to close the path. You can using the pen tool. This may be the simplest way. There are other ways such as making the path an outline, but usually you wont get a fill. If you want a fill but not a black line for example you could hide the line behind another fill. An alternate method is to copy the layer on top of the fill layer and apply a normal path and then change this to an outline.
It all sounds a little complicated but it is a lot easier if all your paths are closed.