This appears to occur very infrequently and at random when a
QNetworkAccessManager is in use. Completely unrelated Qt functions return
QNetworkReply objects, which usually leads to missing attribute exceptions.
Some examples:

I recently observed QWebView.page() return a QNetworkReply object on my Ubuntu
system, with PySide 1.1.0 from the official PPA.

At a similar time, one of my Windows users reported a QPlainTextEdit.document()
returning a QNetworkReply.

I have seen the same issue a few times on my own system with different
originating functions, and get both occasional reports with enough detail
(debug logs) to be sure it is this issue and multiple reports of odd transient
issues which could be attributed to this problem from my users, on both Windows
and Linux systems.

I have also seen the same issue reported by another user on #pyside:
Feb 12 13:31:35 <shevron> I am seeing very strange intermittent errors, where
an object which should be a QWebPage object is actually a QNetworkReply object
(yes I know it sounds wierd) - has anyone encountered something like that?
Feb 12 13:32:27 <shevron> actually now I see it's not the web page, it's the
QWebFrame object belonging to that page
Feb 12 13:33:09 <shevron> what version / OS are you on? I'm on the Ubuntu
packages for 11.10
Feb 12 13:33:21 <shevron> (I guess it's Qt 4.7.4, PySide 1.1.0 from the PPA)

Due to the infrequent nature of this issue, it's proven immensely difficult to
pin down any more debugging information, which I was hoping to obtain before
reporting. The glitch is too infrequent and random to be easily trapped to try
and work out what's happening. If I do find any more information, I'll update
the bug report.