Compilations on OSX machines mysteriously fail to execute system utilities

Occasionally, OSX builds will fail with garbage errors of the form:

11/4/11 6:08:33 PM EST: /bin/sh: /bin/sh: cannot execute binary file
11/4/11 6:08:33 PM EST: make[3]: *** [.obj/release-shared/moc_qcdestyle.o] Error 126

(from http://pulse.test.qt.nokia.com:8080/browse/projects/QtDoc%20master%20Integration/builds/141/logs/stage/macx-g%2B%2B%20developer-build%20cocoa%20OSX%2010.6%20x64/raw/true/ , http://codereview.qt-project.org/7866 )

/bin/sh is the most commonly referenced tool, but sometimes the error also refers to other core utilities such as rm or ln.

This bug is very difficult to reproduce on demand. Some notes:

• the bug has been observed for several years, across OSX 10.5, 10.6 and 10.7, on high-end (xserve) and low-end (mac mini) machines.
• the bug seems to occur more frequently when the machines are under heavy load, e.g. running `make -j25' on the mac minis causes the error to occur significantly more often than `make -j9'.