I have a faced a scenario, wherein I am not able to know the reason for it.
While trying to display an html page (kept in my local directory), I have created 2 web projects. One with only a jsp and written file I/O code to get the file and writing in the ServletOutputStream. But it didn't work. It gave me IlleagalStateException, saying getOutputStream already called.
However, the other project wherein I had a servlet, with same code. It worked fine.