Not an error exactly, but the Top Link Bar doesn't highlight correctly, i.e. when you click on a subsite, the button is not highlighted.
Rather annoying, but simple to fix.
1. Site Settings
2. Top Link Bar
3. Edit the Link properties
Make sure the link is in relative URL format and includes the page, i.e. /sites/sitename/default.aspx, or /someurl/somesite/mypage.aspx
If you can't edit the link, then delete the button, and recreate it.