It's a bug in IE. There is (was?) a similar bug in Safari. What happens is that the browser does not re-parse all the information it has on the page when loading it from the cache, and so it creates the form control in a different way than the first time. What it boils down to is exactly what you see: you put some non-ASCII character in a field, submit, back, and your characters are mangled or gone. There is nothing you can do to fix it, you can only work around it.
I'd second the suggestion of giving the error together with the form, pre-filled with the submitted values.
