This is fairly minor because the use cases aren't all that common, but when I tried running bundle console while using this gem, I got this error:
$ bundle console
/home/mc/app/releases/2011-05-31_22-51-07Z/vendor/bundle/ruby/1.9.1/gems/validates_existence-0.6.2/lib/validates_existence.rb:3:in `<top (required)>': uninitialized constant ActiveRecord (NameError)
I believe this is because active_record is not listed as a dependency in the gemspec.