is_category() function

My suspicion is that, for some reason, the name “51m series” or slug 51m-series is (or both are) matching is_category( 51 ).

The is_category() function uses the PHP in_array() conditional to match against ID, slug, and name:

if ( in_array( $cat_obj->term_id, $category ) )
     return true;
elseif ( in_array( $cat_obj->name, $category ) )
    return true;
elseif ( in_array( $cat_obj->slug, $category ) )
    return true;

The in_array() conditional appears to be returning true when 51 is found in the name/slug 51m series/51m-series. While both the name and slug conditional checks should be case-sensitive, none of the checks are strict. So the integer 51 is possibly being evaluated as a string against $cat_obj->name, and matching 51m Series (or against $cat_obj->slug, and matching 51m-series)

Possible solutions:

• Use category names/slugs that don’t begin with numbers
• Query explicitly against category slugs rather than against category IDs

This may also be a case where core needs to perform a strict check in in_array(), to differentiate between an ID integer and a name/slug string.