Currently a failed APK install won't mark the build as failed - it likely should. Eg:

[tmp] $ /home/foo/tools/android-sdk/platform-tools/adb -s localhost:55208 install -r benchmark.apk
783 KB/s (62492 bytes in 0.077s)
pkg: /data/local/tmp/benchmark.apk
Failure [INSTALL_FAILED_OLDER_SDK]