For some reason, with Dreamweaver (and maybe other editors), the BOM (Byte Order Mark) is introduced when you have not asked it to be... and only on some pages. It's a bug. Problem is, the bug makes your page invalid to W3C standards. The DW validation tool won't pick this up
Download and install the free Notepad++.
Whenever the W3C validator throws up the BOM error, just open up the file in Notepad++, click on the 'Format' option, tick 'encode in UTF-8 without BOM' and then save your file. The BOM is taken out and your page is valid again.