RVM needs to be updated regularly to know of Ruby versions released since installation (or last update).

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So if you say rvm install 1.9.3, but get an old version (basically anything below 1.9.3-p385 when writing this card), your RVM is outdated. \
Fix that by saying:

rvm get stable

After that, rvm install 1.9.3 should install the latest 1.9.3 version.