here are some interesting stats from google’s “reader” blog:

% of errors	Error description
15.6%	Input claims to be UTF-8 but contains invalid characters.
14.9%	Opening and ending tags mismatch
13.9%	An undefined entity is used (e.g.   in an XML document without importing the HTML set)
7.8%	Documented expected to begin with a start tag, but no < was found
5.7%	Disallowed control characters present
5.5%	Extra content at the end of the document
4.2%	Unterminated entity reference (missing semi-colon)
4.2%	Unquoted attribute value
3.8%	Premature end of data in tag (truncated feed)
3.3%	Naked ampersand (should be represented as &amp;)
2.1%	XML declaration allowed only at the start of the document
1.8%	Namespace prefix is used but not defined
0.75%	Comment not terminated
0.64%	Attribute without value
0.17%	Unescaped < not allowed in attributes values
0.11%	Malformed numerical entity reference
0.11%	Unsupported/invalid encoding
0.10%	Comment must not contain ‘–’
0.10%	Attribute defined more than once
0.07%	Char out of allowed range
0.03%	Comment not terminated
0.02%	Sequence ]]> not allowed in content