"IErrorInfo.GetDescription failed with E_FAIL(0x80004005)". I get this when there are unescaped quotes in the javascript arrays (like SYSTEM_ITEMAUTHORS...). So any matched file with an author field that contains a quote will cause the start++ search startlet to fail.
"Expected hexadecimal digit". When specifying a folder constraint in my search query, the "SYSTEM_ITEMFOLDERPATHDISPLAY" variable gets added to the generated script when the search startlet is run. The problem is that this variable is filled with "\"s rather than the "/"s used everywhere else. So it is a big problem when you have a folder that starts with a "x"... that gets treated like a hex escape sequence, thus the error message.
There may be others... but I think it would just be easier to escape obsessively all characters that can be escaped rather than try and track down every possible problem.