I encountered this problem when I synced my evernote in Mac OS.

Can't synchronize error, Content is not valid.

I googled and found this in the evernote support forum.

One of the Ruby gems replaces the built-in 'tidy' command in OS X (/usr/bin/tidy) with a broken script in the same location. You need to restore the original /usr/bin/tidy command that comes with OS X.

On my 10.6.6 box, that command is 65072 bytes:

~ $ ls -al which tidy
rwxr-xr-x 1 root wheel 65072 Aug 31 2010 /usr/bin/tidy

Once you've fixed that broken part of the OS, Evernote should work ok. You may also want to file a bug with the Ruby folks, who absolutely should not be replacing this file with their own.

It seems like that ruby gems in mac did a mess that replaced the /usr/bin/tidy command unintentionally.
So if your tidy file size does not match 65072 bytes, just copy on from other Mac OS, and chmod it.

sudo gem install railsでこんなエラーが出ちゃいました。

Error installing rails bundler requires RubyGems version >= 1.3.6

解決策は

sudo gem update --system

pdating RubyGems
Updating rubygems-update
Successfully installed rubygems-update-1.6.1
Updating RubyGems to 1.6.1
Installing RubyGems 1.6.1
RubyGems 1.6.1 installed

=== 1.6.1 / 2011-03-03

Bug Fixes:

#  Installation no longer fails when a dependency from a version that won't be
  installed is unsatisfied.
#  README.rdoc now shows how to file tickets and get help.  Pull Request #40 by
  Aaron Patterson.
#  Gem files are cached correctly again.  Patch #29051 by Mamoru Tasaka.
#  Tests now pass with non-022 umask.  Patch #29050 by Mamoru Tasaka.


------------------------------------------------------------------------------

RubyGems installed the following executables:
    /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/gem

を実行した後にsudo gem install railsでrailsをインストールすればOKです。