Did you save a copy of the pages before you made changes to them ? you could always compare the old and new code, or use a text/html editor with 'syntax highlighting' that may show unclosed tags etc.. in your page.
Better yet, quit using Front Page. The fact that it is a Microsoft product, should alert you that it will create horrid bloated and broken HTML code and cause problems for non MS browsers..
There are other options..
Blue Fish Editor
Check the Linux Questions: Members Choice Awards
for some other options.