Modified version of image_get_intermediate_size (WordPress, media.php) which checks for an exact match (width and height) before attempting to find a matching width OR height. This fixes the function's behavior in some cases, but a better solution is to add proper aspect handling to the function.

image_get_intermediate_size

/**
 * Retrieve the image's intermediate size (resized) path, width, and height.
 *
 * The $size parameter can be an array with the width and height respectively.
 * If the size matches the 'sizes' metadata array for width and height, then it
 * will be used. If there is no direct match, then the nearest image size larger
 * than the specified size will be used. If nothing is found, then the function
 * will break out and return false.
 *
 * The metadata 'sizes' is used for compatible sizes that can be used for the
 * parameter $size value.
 *
 * The url path will be given, when the $size parameter is a string.
 *
 * If you are passing an array for the $size, you should consider using
 * add_image_size() so that a cropped version is generated. It's much more
 * efficient than having to find the closest-sized image and then having the
 * browser scale down the image.
 *
 * @since 2.5.0
 * @see add_image_size()
 *
 * @param int $post_id Attachment ID for image.
 * @param array|string $size Optional, default is 'thumbnail'. Size of image, either array or string.
 * @return bool|array False on failure or array of file path, width, and height on success.
 */
function image_get_intermediate_size($post_id, $size='thumbnail') {
	if ( !is_array( $imagedata = wp_get_attachment_metadata( $post_id ) ) )
		return false;

	// get the best one for a specified set of dimensions
	if ( is_array($size) && !empty($imagedata['sizes']) ) {

		foreach ( $imagedata['sizes'] as $_size => $data ) {
			// already cropped to width and height; so use this size
			if ( ( $data['width'] == $size[0] && $data['height'] == $size[1] ) ) {
				$file = $data['file'];
				list($width, $height) = image_constrain_size_for_editor( $data['width'], $data['height'], $size );
				return compact( 'file', 'width', 'height' );
			}
		}

		foreach ( $imagedata['sizes'] as $_size => $data ) {
			// already cropped to width or height; so use this size
			if ( ( $data['width'] == $size[0] && $data['height'] <= $size[1] ) || ( $data['height'] == $size[1] && $data['width'] <= $size[0] ) ) {
				$file = $data['file'];
				list($width, $height) = image_constrain_size_for_editor( $data['width'], $data['height'], $size );
				return compact( 'file', 'width', 'height' );
			}
			// add to lookup table: area => size
			$areas[$data['width'] * $data['height']] = $_size;
		}
		if ( !$size || !empty($areas) ) {
			// find for the smallest image not smaller than the desired size
			ksort($areas);
			foreach ( $areas as $_size ) {
				$data = $imagedata['sizes'][$_size];
				if ( $data['width'] >= $size[0] || $data['height'] >= $size[1] ) {
					// Skip images with unexpectedly divergent aspect ratios (crops)
					// First, we calculate what size the original image would be if constrained to a box the size of the current image in the loop
					$maybe_cropped = image_resize_dimensions($imagedata['width'], $imagedata['height'], $data['width'], $data['height'], false );
					// If the size doesn't match within one pixel, then it is of a different aspect ratio, so we skip it, unless it's the thumbnail size
					if ( 'thumbnail' != $_size && ( !$maybe_cropped || ( $maybe_cropped[4] != $data['width'] && $maybe_cropped[4] + 1 != $data['width'] ) || ( $maybe_cropped[5] != $data['height'] && $maybe_cropped[5] + 1 != $data['height'] ) ) )
						continue;
					// If we're still here, then we're going to use this size
					$file = $data['file'];
					list($width, $height) = image_constrain_size_for_editor( $data['width'], $data['height'], $size );
					return compact( 'file', 'width', 'height' );
				}
			}
		}
	}

	if ( is_array($size) || empty($size) || empty($imagedata['sizes'][$size]) )
		return false;

	$data = $imagedata['sizes'][$size];
	// include the full filesystem path of the intermediate file
	if ( empty($data['path']) && !empty($data['file']) ) {
		$file_url = wp_get_attachment_url($post_id);
		$data['path'] = path_join( dirname($imagedata['file']), $data['file'] );
		$data['url'] = path_join( dirname($file_url), $data['file'] );
	}
	return $data;
}