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The HTML generated by system_preprocess_block (in system.module) when outputting menu blocks contains an attribute called 'classes' that contains 'block-menu'.
This was probably created by somebody accidentally writing out 'class' as 'classes' in the attributes array.
<div id="block-footer" class="block block-system contextual-region" role="navigation" classes="block-menu">
I will attach a patch after I get an issue number.