There's a funny post at The Apple Blog today that points out a few typos that give Mac OS X Leopard's built-in spell checker a bit of trouble.
For instance, the error "instrucitons" suggests "isntrucitons" as well as "instructions", and "applicaticataion" when "applicatication" is typed instead of "application." There are a few more in their list.
I've noticed that my iPhone always suggests "he'll" when I want to type "hell" (yes, I type "hell" enough to have noticed this).
Of course, this is simple to fix. You can correct these errors for good by right-clicking (or Control-click) on the correct spelling and select "Learn spelling" from the contextual menu. Use this same method to add uncommon words, like surnames, etc.