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.
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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..
Quanta +
Blue Fish Editor
Mozilla Composer
NVU
Check the
Linux Questions: Members Choice Awards for some other options.