Solr Indexing Error showing "EntityMalformedException: Missing bundle property on entity of type taxonomy_term"
Drupal tries to index an invalid item and throws this error. 
To find out what document is causing the error, you can implement the hook_apachesolr_index_document_build hook with a watchdog statement inside.

This is from apachesolr.api.php


/**
 * Build the documents before sending them to Solr.
 * The function is the follow-up for apachesolr_update_index
 *
 * @param integer $document_id
 * @param array $entity
 * @param string $entity_type
 */
function hook_apachesolr_index_document_build(ApacheSolrDocument $document, $entity, $entity_type, $env_id) {

}


My guess is that the last item in your dblog will be the offending item.