When you try to deploy a pipeline with an invalid schedule, you get an error saying "The cron expression <cron> is invalid". You can do this by going to Schedule, and inputting an invalid schedule in the Advanced section. See attached image and pipeline for an example.
However, the pipeline still gets deployed (even though you stay in the studio). When you try to deploy again, it throws an error saying the pipeline exists.
If the cron expression is invalid and the UI gets an error, then the pipeline should not deployed.