According to w3c, the <td> tags must be always located inside <tr> tag.
An overlapping <td> outside <tr> will always invalidate a documents.

However, when you validate a page with that kind of error, W3C validator will tell you have missing <table> start tag instead. Regardless of the presence of the <table> tag.

To be exact, w3c will generate Error 66,
"document type does not allow element "tr" here; assuming MISSING "table" start-tag"

and

Error 70
"70: end tag for table omitted, but OMITTAG NO was specified"

This could be confusing to web developer/designer relying on W3C validator to produce standard compliant webpage.

Perhaps, the more proper error for a <td> outside <tr> is Error 73 (end tag for X which is not finished). Error 73 also has clear explanation regarding usage of element which requires a child element.

I apologise if I submit this e-mail to the wrong component, but I do believe this issue needs to be brought up.