diff --git a/src/allmydata/download.py b/src/allmydata/download.py
index 29ceaa79f..8b963fbb6 100644
--- a/src/allmydata/download.py
+++ b/src/allmydata/download.py
@@ -35,14 +35,12 @@ class Output:
         self._segment_number = 0
         self._plaintext_hash_tree = None
         self._crypttext_hash_tree = None
+        self._opened = False
 
     def setup_hashtrees(self, plaintext_hashtree, crypttext_hashtree):
         self._plaintext_hash_tree = plaintext_hashtree
         self._crypttext_hash_tree = crypttext_hashtree
 
-    def open(self):
-        self.downloadable.open()
-
     def write_segment(self, crypttext):
         self.length += len(crypttext)
 
@@ -71,9 +69,13 @@ class Output:
         self._segment_number += 1
         # We're still at 1*segment_size. The Downloadable is responsible for
         # any memory usage beyond this.
+        if not self._opened:
+            self._opened = True
+            self.downloadable.open()
         self.downloadable.write(plaintext)
 
     def fail(self, why):
+        log.msg("UNUSUAL: download failed: %s" % why)
         self.downloadable.fail(why)
 
     def close(self):
@@ -269,7 +271,6 @@ class FileDownloader:
         self._num_needed_shares = d['needed_shares']
 
         self._output = Output(downloadable, d['key'])
-        self._output.open()
 
         self.active_buckets = {} # k: shnum, v: bucket
         self._share_buckets = [] # list of (sharenum, bucket) tuples
diff --git a/src/allmydata/interfaces.py b/src/allmydata/interfaces.py
index f2cb478d7..6582f9946 100644
--- a/src/allmydata/interfaces.py
+++ b/src/allmydata/interfaces.py
@@ -605,7 +605,9 @@ class IDecoder(Interface):
 
 class IDownloadTarget(Interface):
     def open():
-        """Called before any calls to write(), close(), or fail()."""
+        """Called before any calls to write() or close(). If an error
+        occurs before any data is available, fail() may be called without
+        a previous call to open()."""
     def write(data):
         """Output some data to the target."""
     def close():
diff --git a/src/allmydata/test/test_system.py b/src/allmydata/test/test_system.py
index 0961a6129..59b69c728 100644
--- a/src/allmydata/test/test_system.py
+++ b/src/allmydata/test/test_system.py
@@ -14,7 +14,7 @@ from foolscap.eventual import flushEventualQueue
 from twisted.python import log
 from twisted.python.failure import Failure
 from twisted.web.client import getPage
-from twisted.web.error import PageRedirect
+from twisted.web.error import PageRedirect, Error
 
 def flush_but_dont_ignore(res):
     d = flushEventualQueue()
@@ -527,9 +527,12 @@ class SystemTest(testutil.SignalMixin, unittest.TestCase):
             return getPage(base + "download_uri/%s?filename=%s"
                            % (self.mangle_uri(self.uri), "mydata567"))
         d.addCallback(_get_from_bogus_uri)
-        def _got_from_bogus_uri(page):
-            self.failUnlessEqual(page, "problem during download\n")
-        d.addCallback(_got_from_bogus_uri)
+        d.addBoth(self.shouldFail, Error, "downloading bogus URI",
+                  "NotEnoughPeersError")
+
+        # TODO: mangle the second segment of a file, to test errors that
+        # occur after we've already sent some good data, which uses a
+        # different error path.
 
         # download from a URI pasted into a form. Use POST, build a
         # multipart/form-data, submit it. This actualy redirects us to a
diff --git a/src/allmydata/webish.py b/src/allmydata/webish.py
index e35c3ac14..34d1942e4 100644
--- a/src/allmydata/webish.py
+++ b/src/allmydata/webish.py
@@ -1,6 +1,6 @@
 
 from twisted.application import service, strports
-from twisted.web import static, resource, server, html
+from twisted.web import static, resource, server, html, http
 from twisted.python import util, log
 from nevow import inevow, rend, loaders, appserver, url, tags as T
 from nevow.static import File as nevow_File # TODO: merge with static.File?
@@ -322,27 +322,47 @@ class Directory(rend.Page):
 
 class WebDownloadTarget:
     implements(IDownloadTarget)
-    def __init__(self, req):
+    def __init__(self, req, content_type, content_encoding):
         self._req = req
+        self._content_type = content_type
+        self._content_encoding = content_encoding
+        self._opened = False
+
     def open(self):
-        pass
+        self._opened = True
+        self._req.setHeader("content-type", self._content_type)
+        if self._content_encoding:
+            self._req.setHeader('content-encoding', self._content_encoding)
+        # TODO: it would be nice to set the size header here
+
     def write(self, data):
         self._req.write(data)
     def close(self):
         self._req.finish()
-    def fail(self, why):
-        # I think the content-type is already set, and the response code has
-        # already been sent, so we can't provide a clean error indication. We
-        # can emit text (which a browser might interpret as something else),
-        # and if we sent a Size header, they might notice that we've
-        # truncated the data. Keep the error message small to improve the
-        # chances of having our error response be shorter than the intended
-        # results.
-        #
-        # We don't have a lot of options, unfortunately.
 
-        self._req.write("problem during download\n")
+    def fail(self, why):
+        if self._opened:
+            # The content-type is already set, and the response code
+            # has already been sent, so we can't provide a clean error
+            # indication. We can emit text (which a browser might interpret
+            # as something else), and if we sent a Size header, they might
+            # notice that we've truncated the data. Keep the error message
+            # small to improve the chances of having our error response be
+            # shorter than the intended results.
+            #
+            # We don't have a lot of options, unfortunately.
+            self._req.write("problem during download\n")
+        else:
+            # We haven't written anything yet, so we can provide a sensible
+            # error message.
+            msg = str(why.type)
+            msg.replace("\n", "|")
+            self._req.setResponseCode(http.INTERNAL_SERVER_ERROR, msg)
+            self._req.setHeader("content-type", "text/plain")
+            # TODO: HTML-formatted exception?
+            self._req.write(str(why))
         self._req.finish()
+
     def register_canceller(self, cb):
         pass
     def finish(self):
@@ -374,12 +394,8 @@ class Downloader(resource.Resource):
                              static.File.contentTypes,
                              static.File.contentEncodings,
                              defaultType="text/plain")
-        req.setHeader("content-type", type)
-        if encoding:
-            req.setHeader('content-encoding', encoding)
-        # TODO: it would be nice to set the size header here
 
-        d = self._filenode.download(WebDownloadTarget(req))
+        d = self._filenode.download(WebDownloadTarget(req, type, encoding))
         # exceptions during download are handled by the WebDownloadTarget
         d.addErrback(lambda why: None)
         return server.NOT_DONE_YET