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The Ubuntu i386 buildbot was crashing in test_float in the 3.1 branch; it looks as though _Py_dg_dtoa is producing invalid results.
I've made a couple of checkins to try to diagnose the failure (r82752 and r82754); here's some of the resulting output from http://www.python.org/dev//buildbot/builders/i386%20Ubuntu%203.1/builds/870
test_float
Unexpected failure in format_float_short. Arguments: d = 9999, format_code = 101, mode = 2, precision = 3
digits == :
Unexpected failure in format_float_short. Arguments: d = 0.096000000000000002, format_code = 102, mode = 3, precision = 2
digits == :
':' is the ASCII character after '9', so this is a classic case of the digit '9' being rounded up to the next ASCII digit. I don't know why this is happening on this particular buildbot and no others that I've noticed.
This machine is one where double rounding *is* typically a problem (according to its configure output), so it should be using the _Py_{set,get}_387controlword functions to control the FPU precision; perhaps something's going wrong with this step.