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Description of problem:
Invalid access of a grab mask in the server causes the server to send events to the client when the client hasn't registered for it. libXext intercepts these and prints 

  _xgeWireToEvent: Unknown extension 131, this should never happen

where 131 is (in this case) the input extension opcode

Version-Release number of selected component (if applicable):
xorg-x11-server-1.14.2-9.el7

How reproducible:
Not reliably reproducible since it depends on a random bit to be set or not set. In my case I managed to reproduce and debug it by opening a Xephyr window, clicking inside the window and then moving out of the window (causing a leave event to be sent). Again, this depends on whatever the invalid memory location happens to be.

MODIFIED

xorg-x11-server-1.14.2-10.el7 is available in brew

This is also happening in Fedora 20 (alpha)
(up-to-date as of 2013-10-21)

I can reproduce the problem reliably using graphviz and ImageMagick.

Steps to reproduce:
1: dot -Tpng <<< "digraph d { hello }" | strace -o out display -
2: click on graph

libXext-1.3.2-2.fc20.x86_64
xorg-x11-server-Xorg-1.14.3-4.fc20.x86_64
xorg-x11-server-utils-7.7-2.fc20.x86_64
graphviz-2.34.0-2.fc20.x86_64
ImageMagick-6.8.6.3-3.fc20.x86_64

Richard: I haven't ported those patches to F20 yet, but they're upstream in master and in the stable branch, and in rawhide