diff -r 8e129bb6b571 -r b355d9dd43df etwist/test/unittest_server.py
--- a/etwist/test/unittest_server.py	Mon Jun 27 18:38:08 2011 +0200
+++ b/etwist/test/unittest_server.py	Mon Jun 27 18:48:30 2011 +0200
@@ -15,8 +15,12 @@
 #
 # You should have received a copy of the GNU Lesser General Public License along
 # with CubicWeb.  If not, see <http://www.gnu.org/licenses/>.
+
+import os, os.path as osp, glob
+
 from cubicweb.devtools.testlib import CubicWebTC
-from cubicweb.etwist.server import host_prefixed_baseurl
+from cubicweb.etwist.server import (host_prefixed_baseurl, ConcatFiles,
+                                    ConcatFileNotFoundError)
 
 
 class HostPrefixedBaseURLTC(CubicWebTC):
@@ -50,3 +54,30 @@
         self._check('http://localhost:8080/hg/', 'code.cubicweb.org',
                     'http://localhost:8080/hg/')
 
+
+class ConcatFilesTC(CubicWebTC):
+
+    def tearDown(self):
+        super(ConcatFilesTC, self).tearDown()
+        self._cleanup_concat_cache()
+        self.config.debugmode = False
+
+    def _cleanup_concat_cache(self):
+        uicachedir = osp.join(self.config.apphome, 'uicache')
+        for fname in glob.glob(osp.join(uicachedir, 'cache_concat_*')):
+            os.unlink(osp.join(uicachedir, fname))
+
+    def test_cache(self):
+        concat = ConcatFiles(self.config, ('cubicweb.ajax.js', 'jquery.js'))
+        self.failUnless(osp.isfile(concat.path))
+
+    def test_404(self):
+        # when not in debug mode, should not crash
+        ConcatFiles(self.config, ('cubicweb.ajax.js', 'dummy.js'))
+        # in debug mode, raise error
+        self.config.debugmode = True
+        try:
+            self.assertRaises(ConcatFileNotFoundError, ConcatFiles, self.config,
+                              ('cubicweb.ajax.js', 'dummy.js'))
+        finally:
+            self.config.debugmode = False