Unfortunately it is possible to create a recurrent appointment while setting the end-date of the repeat-cycle to an earlier date than the appointment itself.
(e.g.
title: test
Begin: 15.05.2018
recurrent: weekly - end of repetition: 14.05.2018
)
you can save this appointment without any error-message or warning 
There are two problems with this (beside this appointment is senseless in any way):
- this appointment does not show up in the calendar - therefore you are not able to delete it again (only possible solution: if you remember the title, you can find it using the search-function)
- starting from now on you will get an annoying error-message at every synchronization with e.g. owncloud or nextcloud due to the fact that such an appointment is rejected by the cardDAV interface (correct way) - but looking at 1. -> it is hard to find and delete the appointment
please add a check filtering such appointment settings and inform the user and reject saving such appointments.
this would increase the usability of this great program further 
Thank you!
cheers! 