[nrh]

  To fix this problem, we introduced the simple rule that new 
  stable releases of the software must (a) talk to old stable 
  releases and (b) they must support existing apps, without 
  changes. We more or less succeeded with that, so ZeroMQ 
  versions 3.2 and 4.0 work nicely with 2.2 and 2.1, for  
  example.

...and pain ensued.

[PieterH]

Citation needed.

If you have an example of problems caused by forwards compatibility, please provide it.

[cwyers]

The essay that's quoted from spends the next few paragraphs saying basically that.

[PieterH]

You're right. The essay is out of date. In fact the "don't break stable contracts" rule has made life much simpler. It took a while for that to work through. I've corrected the text. Thanks for pointing it out.