I recently moved our site between servers and did a couple minor version updates to our site (6.15 -> 6.19). At the time I thought I tested the site carefully at each step, but this morning we noticed a major detail was missed...search. It worked before migration, but not now.
Right now our search forms refresh whatever page you search from, but do not perform the search itself. So if you use the search box on the home page, it refreshes the home page. If you try it from the search page you just refresh the search page. We did not change any of the templates along the way, and all the references I could find to similar problems were related to templating mistakes which it seems to me should have applied since day 1 (I have found issues like this: http://stackoverflow.com/questions/2887699/drupal-search-box-not-working...). Are there other likely culprits? Best I can tell (I'm double checking with our designer) we didn't update any of the templates related to search, just the CSS, so the templates seems an unlikely cause.
Using Tamper Data I took a look at what's getting posted, and it appears to me to be to be correct. The form gets posted back to the page that created it, it has a form_id of search_block_form. My instinct says it's a server setup issue, but I can't find anything.