Generator results that are in excess of $wgAPIMaxResultSize will cause an error in XML rather than truncating. This seems to be occuring when the Page elements themselves cause the result size to be in excess of the maximum.
For example, setting $wgAPIMaxResultSize to some arbitrarily low value for demonstration, like 100,000, and doing a standard generator=allpages query, instead of results, you get:
<error code="internal_api_error_MWException" info="Exception Caught: Internal error in ApiFormatXml::recXmlPrint: (pages, ...) has integer keys without _element value. Use ApiResult::setIndexedTagName()." xml:space="preserve">
Interestingly, this occurs regardless of what properties you request, and only occurs with gaplimits over a certain value, regardless of the actual result size (though the gaplimit required to cause the error increases as the max result size does). Both XML and JSON will happily return result sets well above the designated result size (which could be considered another bug), but JSON always seems to return results successfully, where XML fails beyond a certain gaplimit.
Since this becomes nearly impossible to replicate with more normal values of $wgAPIMaxResultSize, I'm marking it as trivial.