Solids fail, that's what they do.
All those things that you have been picking while building your model, Solidworks is remembering. If something is removed then this causes an error. These errors can quickly be resolved by looking at the feature manager tree and looking for indicators which is covered in the online manual. You have to learn these.
In an overdefined situation, typically a sketch will give you an indicator or an assembly will show you an error. The easiest way to fix these errors is to remove one constraint at a time til you understand why it's busted. In the future there's some technology that solidworks is developing that will help solve over defined designs.