Steps to reproduce:

Since one of the latest updates to version 120.3.0 (release) of Thunderbird the calendar view was completely broken. Some events were missing and some months didn't show any events at all in the monthly calendar view.

After some research I found the issue with one of the events that I created with an older version of Thunderbird back in 2018.

This event is configured in that way that it should be repeated every last day of the month AND the 31st day of every month. I attached an .ics file with this event only to recreate the defect. Here is an excerpt of the console log:

Error: Same occurrence found twice, protecting you from death by recursion Ical.jsm:7300:15
NS_ERROR_XPC_JAVASCRIPT_ERROR_WITH_DETAILS: [JavaScript Error: "Same occurrence found twice, protecting you from death by recursion" {file: "resource:///modules/calendar/Ical.jsm" line: 7300}]'[JavaScript Error: "Same occurrence found twice, protecting you from death by recursion" {file: "resource:///modules/calendar/Ical.jsm" line: 7300}]' when calling method: [calIRecurrenceRule::getOccurrences] CalRecurrenceInfo.jsm:556

Steps to recreate:

1. Import the ics-file to any calendar.
2. Open the monthly calendar view.
3. In months with 31 days like September or December no events at all are shown from the very same calendar including the broken event. The broken event is never shown in any month.

I know that the configuration of the event makes no sense and I changed it. Now it only repeats every last day of the month and it works like a charm. But since it worked in earlier versions and events like this can be created it should still work.

Thank you already in advance!

Actual results:

In months with 31 days like September or December no events at all are shown from the calendar with the broken event. The broken event is never shown in any month.

Expected results:

The event and every other event should still be shown or an error dialog should be shown to the user. This worked in earlier versions (tested with version 91).

Error log in the console

xref bug 1789437