How to reproduce:
1. Open attached project "case_1219404-menu_item.zip" and scene "PlayScene"
2. In Hierarchy window, select "Directional Light" object
3. In Inspector window, Test (Script) -> Colors -> click on "Element 0" and select "Test 20" (reproduces with any element and any test)
In the attached project, there's a Test script under the directional light with 2000 elements. If I try to change an element with a high index number the warnings are thrown.

Expected result: no warnings are thrown, the element can be selected and chosen
Actual result: a warning is thrown "Failed to insert item. Menu item insert limit is reached. Name: Test 1001, Command: 1001",
AND the Test -> Colors -> elements cannot be selected/chosen until the Editor is restarted

Reproducible with: 2017.4.39f1, 2018.4.20f1, 2019.3.8f1, 2020.1.0b4, 2020.2.0a5