https://gerrit.wikimedia.org/r/#/q/6967a5cd3a56df9d368aabf6dc3232846a4e4136 caused an error on some VMs in WMCS.
Exactly why it caused the trouble is mysterious to me because the selector expression looks fine to me, but in some cases "false" was inserted for the filename. In all cases I found so far, the puppet version was 4.8.2, but I saw this on Jessie and Stretch VMs that use pinning.
Pins are extensively used on WMCS systems, but here was a good example:
Error: Could not retrieve catalog from remote server: Error 500 on SERVER: Server Error: Evaluation Error: Error while evaluating a Resource Statement, Evaluation Error: Error while evaluating a Resource Statement, Duplicate declaration: File[/etc/apt/preferences.d/false] is already declared in file /etc/puppet/modules/apt/manifests/pin.pp:17; cannot redeclare at /etc/puppet/modules/apt/manifests/pin.pp:17 at /etc/puppet/modules/apt/manifests/pin.pp:17:5 at /etc/puppet/modules/profile/manifests/wmcs/services/toolsdb_apt_pinning.pp:6 on node clouddb1001.clouddb-services.eqiad.wmflabs Warning: Not using cache on failed catalog Error: Could not retrieve catalog; skipping run
Hopefully that helps track how/why this may or may not happen again or so you can enable the feature you were after more effectively.