I have been using WP for a few years, but I'm not a frequent user and not a PHP programmer. So please pardon me if this question is "common knowledge" for most of you.
Today I installed WP 2.6.1 on a new site and I installed two themes from the Most Popular list, namely Cloudy 1.6 and Blue Mist.
After writing a post, and still being logged in as the admin, I tried to write a comment to the post I just made, but the browser just forwarded me to http://secondnorway.net/wp-comments-post.php and displayed a blank page. The comment was not saved!
I instantly knew that such a vital feature could not have passed undetected through the WP release quality control, so I reverted back to the default theme, and now making comments worked fine.
Since these two themes are on the Most popular list with several tousands of downloads, I am sure someone else has faced this issue too, and come up with a solution :) Please reply if you have the solution.