何かの拍子で 2.3.0 に旧バージョンの did_you_mean を入れていたらしい。。。
$ ruby -v -e 'def hi; hi; end; hi'
ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-darwin13]
/Users/koic/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/interception-0.5/lib/cross_platform.rb:66:
warning: method redefined; discarding old start
/Users/koic/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/interception-0.5/lib/interception.rb:113: w
arning: previous definition of start was here
/Users/koic/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/interception-0.5/lib/cross_platform.rb:74:
warning: method redefined; discarding old stop
/Users/koic/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/interception-0.5/lib/interception.rb:117: w
arning: previous definition of stop was here
/Users/koic/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/did_you_mean-0.10.0/lib/did_you_mean/finder
s/method_finder.rb:32: warning: assigned but unused variable - label
$ gem uninstall did_you_mean
Successfully uninstalled did_you_mean-0.10.0
$ ruby -v -e 'def hi; hi; end; hi'
ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-darwin13]
-e:1:in `hi': stack level too deep (SystemStackError)
from -e:1:in `hi'
from -e:1:in `hi'
from -e:1:in `hi'
from -e:1:in `hi'
from -e:1:in `hi'
from -e:1:in `hi'
from -e:1:in `hi'
from -e:1:in `hi'
... 11901 levels...
from -e:1:in `hi'
from -e:1:in `hi'
from -e:1:in `hi'
from -e:1:in `<main>'