I wanted to move away from the --with-pydebug to a normal python and I failed with:

# emerge dev-lang/python:2.7

 * IMPORTANT: 11 news items need reading for repository 'gentoo'.
 * Use eselect news to read news items.

Calculating dependencies... done!
Debug memory block at address p=0xa7f5900: API 'o'
    80 bytes originally requested
    The 7 pad bytes at p-7 are not all FORBIDDENBYTE (0xfb):
        at p-7: 0xfb
        at p-6: 0xfb
        at p-5: 0xfa *** OUCH
        at p-4: 0xfb
        at p-3: 0xfb
        at p-2: 0xfb
        at p-1: 0xfb
    Because memory is corrupted at the start, the count of bytes requested
       may be bogus, and checking the trailing pad bytes may segfault.
    The 8 pad bytes at tail=0xa7f5950 are FORBIDDENBYTE, as expected.
    The block was made by call #21242094 to debug malloc/realloc.
    Data at p: 73 00 00 00 79 00 00 00 ... 67 00 00 00 00 00 00 00
Fatal Python error: bad leading pad byte
Aborted (core dumped)
#

Great. So, what can we gather now from this? It got overwritten within few dozens of seconds at most since I started emerge. It is in a different memory region (so no memory module HW bug). I am attaching the resolved stacktrace just in case ...