TYPO3 Core

With our "seminars" extension, we have the following problem:

The record type "seminar" uses tabs in the BE. In one of the tabs, we use IRRE. When an IRRE relation is added, all tabs starting from the third disappear, and their contents are moved to the second tab.

Steps to reproduce:
1. Install the attached extension (a stripped-down version of the seminars extension).
2. In a system folder, create a seminar record (using the list view).
3. Edit the record and set a title (in the first tab). See that there are 4 tabs.
4. Move to the second tab. See that it contains only one entry.
5. Save the record without closing.
6. In the second tab, add a new time slot (the IRRE relation).
7. Save the record without closing.

Expected result:
There still are 4 tabs, and the second tab still contains one entry.

Actual results:
There are only 2 tabs left (the first tabs and the second tab (which has the IRRE element on it)). All elements from the third and fourth tab are moved to the second tab.

Removing the IRRE relation and saving makes the tabs reappear.

This problem does not occur with TYPO3 4.1.5, but with the current 4.2 branch. If I remember correctly, this regression occurred sometime in early 2008.
(issue imported from #M7634)