Note on NTP jumps

README.markdown

@@ -1,6 +1,6 @@
 # An introduction to distributed systems
 
-Copyright 2014, 2016 Kyle Kingsbury
+Copyright 2014, 2016, 2017 Kyle Kingsbury; Jepsen, LLC.
 
 This outline accompanies a 12-16 hour [overview class on distributed systems
 fundamentals](http://jepsen.io/training.html). The course aims to introduce
@@ -188,6 +188,8 @@ Lamport, 1987:
   - Caveat: By *centuries*
     - NTP might not care
     - http://rachelbythebay.com/w/2017/09/27/2153/
+  - Caveat: NTP can still jump the clock backwards (default: delta > 128 ms)
+    - https://www.eecis.udel.edu/~mills/ntp/html/clock.html
   - Caveat: POSIX time is not monotonic by *definition*
     - Cloudflare 2017: Leap second at midnight UTC meant time flowed backwards
     - At the time, Go didn't offer access to CLOCK_MONOTONIC