Travis CI failed when building an R package with the following error that I've never seen before - and one that seems to be related to the gpg key not being retrieved (instead of the typical errors in my code :)).
Installing R
0.46s$ sudo add-apt-repository "deb http://cloud.r-project.org/bin/linux/ubuntu $(lsb_release -cs)/"
60.08s$ sudo apt-key adv --keyserver ha.pool.sks-keyservers.net --recv-keys E298A3A825C0D65DFD57CBB651716619E084DAB9
Executing: /tmp/tmp.Cp93xmp8GR/gpg.1.sh --keyserver
ha.pool.sks-keyservers.net
--recv-keys
E298A3A825C0D65DFD57CBB651716619E084DAB9
gpg: requesting key E084DAB9 from hkp server ha.pool.sks-keyservers.net
gpgkeys: key E298A3A825C0D65DFD57CBB651716619E084DAB9 can't be retrieved
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0
The command "sudo apt-key adv --keyserver ha.pool.sks-keyservers.net --recv-keys E298A3A825C0D65DFD57CBB651716619E084DAB9" failed and exited with 2 during .
Your build has been stopped.
More is on Travis here