When you get some results in this demo (http://extjs.com/examples/forms/forumsearch.html) and you explicitly put in the toolBar the page you want to display, you get wrong results.
I found out (with FireBug), that when calling the json generator script, http parameter "query" is missing.
Here is what i got (http parameters) going to page 2 using arrow:
and here is what i got using explicitly page number 2:
"query" is missing and that's why i get more than 250 000 results (when searching for "java") instead of some 2000. The same error (not sending parameter "query") happens also when you click refresh in the toolbar.