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Free Software but not Open Source
It is possible for software to be Free Software (in the sense of GPL version 2 compatible), and yet not satisfy the requirements of the Open Source Initiative for being an Open Source license. This is an obscure corner case in the GPL, because people usually (not always) mean Free Software when they say “Open Source” — stressing a technical detail that is a prerequisite for Freedom over Freedom itself.
The relevant bit of the GPLv2 is clause 8:
8. If the distribution and/or use of the Program is restricted in certain countries either by patents or by copyrighted interfaces, the original copyright holder who places the Program under this License may add an explicit geographical distribution limitation excluding those countries, so that distribution is permitted only in or among countries not thus excluded. In such case, this License incorporates the limitation as if written in the body of this License.
You could write GPLv2 licensed software whose distribution to the United States is prohibited, for instance. This clause allowing additional restrictions based on geography has not survived in the GPL version 3.
In any case, for a GPLv2 plus geographical restriction license, the problematic requirement is requirement 5, No Discrimination Against Persons or Groups, formulated as: The license must not discriminate against any person or group of persons. Clearly restricting a GPLv2 licensed product to a certain geographical area discriminates against a specific group (i.e. those outside that area).
I’m told — but have not verified — that there are also two Open Source licenses that are not Free Software (i.e. the converse of the compatibility issue pointed out here). I’m also told that they are used by one project each, so it’s not a huge burden on the Free Software community.