If you take a long time entering a post, it might cause your session to time out, in which case, the forum will ask you to log in again to verify your credentials. If you enter your login information, your post *ought* to still be intact as long as you don't navigate away to a different page.
You can prevent this from happening by checking the "Remember Me" option when you log in. This will use browser cookies to keep you logged into the site. If you explicitly log out (by clicking on "Log Out"), those cookies will be cleared and you will need to re-enter your login information the next time you visit.
Hope that helps.