There are some problems we all seem to "find;" this section documents a few of them. [Remember to always search before posting on the forums.]
Tables messed up, images not showing, other "strange" problems with HTML - I think every Drupal user finds this problem. Drupal defaults to filtered HTML; that is, only certain tags are allowed. Further, that input format also breaks long lines of text. The fix is real easy: switch to the "Full HTML" input format. I make that the default for administrators (like me). Note that you may still want the URL Filter and Code Filter modules turned on for this input format; they are not defaults.
Help, I turned on Site Maintenance, now I can't login! - About 3 out of 4 Drupal users have done this to themselves (including me). You can still log in with
http://www.example.com/?q=user. Note that "user" is exactly that - do not put your username there.