Reported on EnWiki by John Broughton:
Steps to Reproduce:
(1) Click the "transclusion" icon
(2) In the lower left of the resulting dialog box, hover over the "+" sign until the bracket pair ("[ ]") is visible; click on that.
(3) Click on "[ ] Content"
(4) Click on "Apply changes"
Now none of the "insert" icons work, and a number of other icons on the tool bar also don't work. (The user guide is silent as to what these brackets are ''supposed'' to do.)
I can confirm the same issue also occurs for me on Firefox 22, Monobook interface.