ruby gem error: “ERROR: While executing gem … (ConcurrencyError)”

On Windows 7 running ‘gem install [gem name]” I was getting this cryptic error:

ERROR:  While executing gem … (ConcurrencyError)
Detected invalid array contents due to unsynchronized modifications with concurrent users

The only post I found that mentions this exact error was in the comments for this post about Neo4j on Ruby, and specifically getting this error with jruby 1.6.0.-RC2. I was using 1.6.0-RC1. I downloaded the latest version ( and replaced my older version, and that fixed it.