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> My best guess is that you somehow ended up in a situation where the math 
> module was using the x87 FPU for floating-point, while the interpreter 
> core was using SSE2. Is this possible?
> 
That would be the reverse, since this occurred on a 32-bit build, i.e.
the interpreter core was using x87. But I don't understand how a 64-bit
module could be loaded by a 32-bit executable (I did check that
sys.maxsize was 2**31 - 1).
> fsum is a bit broken on systems with double rounding problems. So
there's 
> a pair of lines in testFsum that look like:
> 
> if 1e16+2.0 != 1e16+2.9999:
> return
Wouldn't it be a problem with stale pyc files then? The result of each
addition is stored as a constant when the code is compiled. Note how the
constants arrays differ:
64-bit:
>>> def f():
... return 1e16+2.9999
... 
>>> dis.dis(f)
 2 0 LOAD_CONST 3 (10000000000000002.0) 
 3 RETURN_VALUE 
>>> zlib.crc32(marshal.dumps(f.__code__.co_consts))
2292868100
32-bit:
>>> def f(): 
... return 1e16+2.9999
... 
>>> dis.dis(f) 
 2 0 LOAD_CONST 3 (10000000000000004.0) 
 3 RETURN_VALUE 
>>> zlib.crc32(marshal.dumps(f.__code__.co_consts))
103113703