I'm with you on that one. I use Homesite and I've now developed a "validate button" reflex after I do any editing at all.
I wish that the page builders for many major sites would be required to do it as well. It's the stupid errors, copy/paste mistakes etc that can really do you in over time.
I'm certain that, although Googlebot can accommodate "some" errors, there are certain types of errors (like a missing ">"), and perhaps a threshhold of total errors as well, that make part or all of a page impossible for them to index.