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Here's an example that uses only math library functions and float literals, presumably with a simpler code path:
Python 2.7.3 (default, Jan 2 2013, 16:38:11)
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import math
>>> math.exp(-2.0)
0.1353352832366127
>>> math.ceil(1.0)
1.0
>>> math.exp(-2.0)
0.23902241864785234
By the way, the equivalent C program (using exp() and ceil()) on the same platform behaves correctly, so it's not an obvious libc or fpu problem.