It is typical for distributions to cripple packages.

There is no "zlib-dev" per se. Zlib is a whole package, and 
distributions cripple it for no valid reason whatsoever in the year 
2011. (My hdd has 300 GB, I never need crippled packages.)

I just tried to install rspec:

gem install rspec


Fetching: rspec-core-2.6.4.gem (100%)
Fetching: diff-lcs-1.1.3.gem (100%)
Fetching: rspec-expectations-2.6.0.gem (100%)
Fetching: rspec-mocks-2.6.0.gem (100%)
Fetching: rspec-2.6.0.gem (100%)
Successfully installed rspec-core-2.6.4
Successfully installed diff-lcs-1.1.3
Successfully installed rspec-expectations-2.6.0
Successfully installed rspec-mocks-2.6.0
Successfully installed rspec-2.6.0
5 gems installed
Installing ri documentation for rspec-core-2.6.4...
Installing ri documentation for diff-lcs-1.1.3...
Installing ri documentation for rspec-expectations-2.6.0...
Installing ri documentation for rspec-mocks-2.6.0...
Installing ri documentation for rspec-2.6.0...
Installing RDoc documentation for rspec-core-2.6.4...
Installing RDoc documentation for diff-lcs-1.1.3...
Installing RDoc documentation for rspec-expectations-2.6.0...
Installing RDoc documentation for rspec-mocks-2.6.0...
Installing RDoc documentation for rspec-2.6.0...


And it worked for me. I don't have a crippled zlib though, as I compile 
everything from source (using ruby scripts).

You should ask your distribution why it wants to kill your time by 
crippling packages like zlib.

I do however have to say that something like:

 "incorrect header check"

Is NOT a good error message.

So rubygems guys, make the error response better. What the heck is an 
"incorrect header check"? Be more clear on that!

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