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From http://buildbot.python.org/all/builders/AMD64%20Debian%20root%203.x/builds/2436:
python: Python/pytime.c:633: pymonotonic_new: Assertion `!last_set || last <= *tp' failed.
Fatal Python error: Aborted
Full log is here: http://buildbot.python.org/all/builders/AMD64%20Debian%20root%203.x/builds/2436/steps/test/logs/stdio 

This buildbot runs in a VM. I need more information on the host (machine running the VM): OS, OS version, kernel version, hardware virtualization?, version of qemu/kvm?, etc.
It's probably a bug in the virtualization.
In the PEP 418, we decided to _not_ handle this error (monotonic clock running backward).
Maybe we should document the bug and remove the assertion (it only exists when Python is compiled in debug mode).
Note: I already saw this assertion error on the same buildbot.
http://buildbot.python.org/all/builders/AMD64%20Debian%20root%203.x/builds/2436/steps/test/logs/stdio
== CPython 3.6.0a0 (default:2825c87d3f72, Jul 25 2015, 01:29:19) [GCC 4.7.2]
== Linux-3.2.0-4-amd64-x86_64-with-debian-7.7 little-endian
== hash algorithm: siphash24 64bit
== /root/buildarea/3.x.angelico-debian-amd64/build/build/test_python_27852
http://buildbot.python.org/all/buildslaves/angelico-debian-amd64
Slave information
* Buildbot-Slave 0.8.6p1
* Debian AMD64 running tests as root - VM with two Intel i5 cores

The host is running Debian Jessie (newer than the Debian Wheezy of the VM).
Linux sikorsky 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt9-3~deb8u1 (2015年04月24日) x86_64 GNU/Linux
What info are you after re hardware virtualization?
VirtualBox 4.3.28 r100309 manages the VM.
Any other information that would help?

Where's this issue heading? The buildbots seem to be green now, does that imply that this is fixed?

Chris Angelico added the comment:
> Where's this issue heading? The buildbots seem to be green now, does that imply that this is fixed?
The issue still exist, it's just that the issue occurs randomly and is
very rare (seen twice in 6 months...).

Ah. It's one of *those* bugs. Got it.

Hum, I'm not sure that I was clear: in fact, the assertion is only
useful for me, because I'm curious to know bugs in implementations of
monotonic clocks. In the case of this buildbot, it may be a bug in
qemu, less likely in the linux kernel.
I don't think that Python should workaround bugs in the OS (kernel).

IMHO the best to do with this issue is to document that Python doesn't ensure that the clock is monotonic, but only exposes what the OS provides, and then remove the assertion.

It occurred on more time on the same buildbot:
http://buildbot.python.org/all/builders/AMD64%20Debian%20root%203.x/builds/2569/steps/test/logs/stdio 

New changeset 4c90b4a4fffa by Victor Stinner in branch '3.5':
Issue #24707: Remove assertion in monotonic clock
https://hg.python.org/cpython/rev/4c90b4a4fffa 

I removed the assertion. Thanks for your report!