I think the biggest problem is people independently inventing things. Late last year, I thought for sure the box shadow drawing technique I spent several days developing from first principles was original. Turns out that, of course, it wasn't: the technique had been described a year or two before that and was written up on a blog.

Carmack's Reverse is probably the most well-known example of this, also in the graphics field.

It may still be optimal in some macroeconomic sense to encourage two people who independently invent an idea to race to patent it and to have the winner of the race sue the loser. But I think most people would consider this scenario unfair.