Known incompatibilites from Ruby 1.9.3 to 2.2.3

(Note: Crossposted at

after not having got a response here).

I have - for technical reason - to start the development based on Ruby
1.9.3, but expect (at least hope) that we can, during the course of the
project, switch to Ruby 2.2.x. Therefore I will try hard not to
introduce any code, which would be broken when upgrading to the higher

Apart from possible incompatibilities in Gems, is there something on the
Ruby core, respectively the standard libraries, which is not supported
anymore in the current Ruby, or is functionally different?