If you try to enter a cron expression in the Web UI like H/2 * * * * * you will get an error.
If you use job-dsl construct like
the job will be created without any issues. The incorrect values will be there in the config.xml, but if you open the Web UI you will not see the trigger at all. There is nothing to be seen in jenkins log. This won't be flagged in the Manage Old Data monitor either.
Best solution would be too tighten validation for the executed job-dsl to not allow such values. If that's not possible it would be nice to at least be able to see a notification about the error somewhere.