Duplicate Nested Errors in Processing

Earlier today I came across a "Duplicate Nested" error in Processing. I spent a frustrating 15 minutes going through my code, trying to figure out what was wrong. When I did some research on the internet I found out that this error essentially meant that I had "defined a class inside a sketch of same name as [my] class" - thanks PhiLho for the tip.

This was true, I had saved the second tab on the IDE as the name of an object I created. To solve the problem all I changed the name. At first it didn't work, that is until I restarted the application. So keep an eye out for this error because if you never heard of it before you will probably be scratching your head when it happens, like I was...