diff --git a/scrapy/core/engine.py b/scrapy/core/engine.py
index 1fdaf476f..a26206db6 100644
--- a/scrapy/core/engine.py
+++ b/scrapy/core/engine.py
@@ -137,7 +137,8 @@ class ExecutionEngine(object):
         if request:
             dwld = mustbe_deferred(self.download, request, spider)
             dwld.chainDeferred(deferred).addBoth(lambda _: deferred)
-            dwld.addErrback(log.err, "Unhandled error on engine._next_request")
+            dwld.addErrback(log.err, "Unhandled error on engine._next_request()",
+                spider=spider)
             return dwld
 
     def spider_is_idle(self, spider):
@@ -173,7 +174,7 @@ class ExecutionEngine(object):
         # next core refactoring.
         #schd.addErrback(log.err, "Error on engine.crawl()")
         schd.addBoth(self.scraper.enqueue_scrape, request, spider)
-        schd.addErrback(log.err, "Unhandled error on engine.crawl()")
+        schd.addErrback(log.err, "Unhandled error on engine.crawl()", spider=spider)
         schd.addBoth(lambda _: self.next_request(spider))
 
     def schedule(self, request, spider):
diff --git a/scrapy/core/scraper.py b/scrapy/core/scraper.py
index 8a0618810..5bbd480a7 100644
--- a/scrapy/core/scraper.py
+++ b/scrapy/core/scraper.py
@@ -186,7 +186,8 @@ class Scraper(object):
         if isinstance(spider_failure.value, IgnoreRequest):
             return
         elif spider_failure is propagated_failure:
-            log.err(spider_failure, 'Unhandled error propagated to spider and wasn\'t handled')
+            log.err(spider_failure, 'Unhandled error propagated to spider', \
+                spider=spider)
             return # stop propagating this error
         else:
             return spider_failure # exceptions raised in the spider code
diff --git a/scrapy/log.py b/scrapy/log.py
index c2cf4d089..71897ffee 100644
--- a/scrapy/log.py
+++ b/scrapy/log.py
@@ -87,7 +87,11 @@ def err(_stuff=None, _why=None, **kwargs):
     domain = kwargs.pop('domain', None)
     spider = kwargs.pop('spider', None)
     component = kwargs.pop('component', BOT_NAME)
-    kwargs['system'] = domain or spider.domain_name if spider else component
+    if domain is not None:
+        import warnings
+        warnings.warn("'domain' argument of scrapy.log.err() is deprecated, " \
+            "use 'spider' argument instead", DeprecationWarning, stacklevel=2)
+    kwargs['system'] = domain or (spider.domain_name if spider else component)
     if _why:
         _why = unicode_to_str("ERROR: %s" % _why)
     log.err(_stuff, _why, **kwargs)