When a change happens in Wikibase that breaks something in Lexeme we don't know until it has already been merged.
This is a problem because we could plausibly have a release cut at this stage. AFAICT no additional CI jobs are run for Lexeme when the release branch is cut so I think these breaks could get as far as wikidata.org
An example of a (minor) breaking change is: https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/Wikibase/+/448005/
Which caused breaks in some of the Lexeme tests such as: https://integration.wikimedia.org/ci/job/mwext-testextension-php70-composer-jessie/1083/console
We fixed this in https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/WikibaseLexeme/+/448016/
In order to prevent this happening we may want to run the Lexeme test suite on changes to Wikibase. One question is should this test be voting? If so does it put us in catch-22 and stop any breaking changes (even desired ones) in Wikibase.
We may also want to run a nightly test to give us a warning.