While investigating #1435 I ran into some other formatting issues. The output isn't necessarily wrong, but it doesn't match CPython either (although I am concerned about the outputting more data than requested potentially causing a buffer overflow).

1 - Outputting more data than requested.
On pyboard:

>>> '{:.4e}'.format(9.99999e-5)
'1.00000e-04'

On CPython:

>>> '{:.4e}'.format(9.99999e-5)
'1.0000e-04'

2 - making the wrong choice about when to use e notation:
pyboard:

>>> '%5.g' % 9.99999
'   10'

CPython:

>>> '%5.g' % 9.99999
'1e+01'

3 - Failing to normalize when rounding:
pyboard:

>>> '%5.e' % 9.99999
'10e+00'

CPython:

>>> '%5.e' % 9.99999
'1e+01'