Certain users suddenly cannot insert components using the page editor

Okay, this was an interresting issue to debug.


One of our customers was having an issue with one (and only one) of their users not being able to insert components on pages using the page editor.


First we tried comparing permissions, role membership with other users (they matched btw.), and nothing helped.


To solve it, we wrote a small aspx page, that changed the current user context to the broken user (to avoid having to ask the user for her password, since she is loaded using the AD module, getting her AD password is a big no-no).


Then, as soon as we had the user context, it was pretty easy to open the page editor, and see for ourselves.


For some reason, if the user has unchecked the Design checkbox, the New Component button is just disabled (hint to UI designes - if you disable something, add a tooltip that tells why it is disabled!).


That also explains why it only happened to her, since it is saved on her Sitecore profile.