What is "marshal data"?

[Soren Ragsdale]

I'm testing some scripts with Python 2.1.1: we're upgrading from
1.5.2.  I've been noticing this error, and the REAL problem is that
I've never heard of "marshal data" before.  This error is not
reproducible: this script is part of a distributed program which runs
on about 200 machines, and four of the scripts produced this error. 
The problem isn't isolated to individual machines:

> /tip/td/python2-irix64/bin/python -u ren_blade.py 44
Traceback (most recent call last):
  File "ren_blade.py", line 4, in ?
    import uh,gh
ValueError: bad marshal data

Possible sources of the problem: "uh.py" is imported from MANY
programs on our network.  It's being imported by Linux and Irix
machines running 1.5.2 and 2.1.1 versions of Python.  Could this be
some sort of race condition, where one platform or version produces a
.pyc file while the other platform or version is also writing the same
.pyc file?