IRB session without ruby-unit in Gemfile:
irb(main):001:0> 4/3 # => 1
irb(main):002:0> (4/3).class # => Fixnum
IRB session with ruby-unit in Gemfile:
irb(main):001:0> 4/3 # => (4/3)
irb(main):002:0> (4/3).class # => Rational
Of course, I can just call (4/3).to_i to get the old result, but I think, introducing the gem to an existing project should not require changes to unrelated code.