Mortar will occasionally jam, and allow you to queue skills without it executing any of them. When it stops being jammed, it fires two #1 shots. The duration of jamming varies, and appears to be extended by continued use of skills while jammed; I tested up to 35 iterations of use of the #1 skill during jamming, and it fired two shots a few seconds after I stopped attempting to fire. It appears to be triggered by rapid use of skills, but it’s difficult to formulate a replication method beyond ‘Use Mortar skills in rapid succession.’ It can even be triggered by simply using Skill #1 repeatedly.