b.diff

diff --git a/scrapy/core/scraper.py b/scrapy/core/scraper.py
index 187c1e9..acbf6ea 100644
--- a/scrapy/core/scraper.py
+++ b/scrapy/core/scraper.py
@@ -103,7 +103,8 @@ class Scraper(object):
             self._scrape_next(spider, slot)
             return _
         dfd.addBoth(finish_scraping)
-        dfd.addErrback(log.err, 'Scraper bug processing %s' % request, \
+        dfd.addErrback(log.err, 'Scraper bug processing %(request)s',
+                       request=request, spider=spider)
             spider=spider)
         self._scrape_next(spider, slot)
         return dfd
@@ -146,7 +147,8 @@ class Scraper(object):
         if isinstance(exc, CloseSpider):
             self.crawler.engine.close_spider(spider, exc.reason or 'cancelled')
             return
-        log.err(_failure, "Spider error processing %s" % request, spider=spider)
+        log.err(_failure, "Spider error processing %(request)s",
+                request=request, spider=spider)
         self.signals.send_catch_log(signal=signals.spider_error, failure=_failure, response=response, \
             spider=spider)
         self.crawler.stats.inc_value("spider_exceptions/%s" % _failure.value.__class__.__name__, \
@@ -205,7 +207,7 @@ class Scraper(object):
                 return self.signals.send_catch_log_deferred(signal=signals.item_dropped, \
                     item=item, spider=spider, exception=output.value)
             else:
-                log.err(output, 'Error processing %s' % item, spider=spider)
+                log.err(output, 'Error processing %(item)s', item=item, spider=spider)
         else:
             logkws = self.logformatter.scraped(output, response, spider)
             log.msg(spider=spider, **logkws)
diff --git a/scrapy/log.py b/scrapy/log.py
index be2c1fd..bc05a50 100644
--- a/scrapy/log.py
+++ b/scrapy/log.py
@@ -1,4 +1,4 @@
-""" 
+"""
 Scrapy logging facility
 
 See documentation in docs/topics/logging.rst
@@ -7,11 +7,11 @@ import sys
 import logging
 import warnings
 
-from twisted.python import log
+from twisted.python import log, _safeFormat
 
 import scrapy
 from scrapy.utils.python import unicode_to_str
- 
+
 # Logging levels
 DEBUG = logging.DEBUG
 INFO = logging.INFO
@@ -131,6 +131,8 @@ def msg(message=None, _level=INFO, **kw):
 def err(_stuff=None, _why=None, **kw):
     kw['logLevel'] = kw.pop('level', ERROR)
     kw.setdefault('system', 'scrapy')
+    if _why is not None:
+        _why = _safeFormat(_why, kw)
     log.err(_stuff, _why, **kw)
 
 def start_from_crawler(crawler):