Crash occurs when the item appears in an inventory or hotbar; otherwise game runs fine even if the item is placed in the world (tested with stone blocks, by loading a new world). Attempted with a gun from Flan's mod as well as Vanilla stone blocks. Gives an overflow error. Attached Both crash logs.

To Duplicate: Enter any game, select any item, use "/armourers setItemAsSkinnable" two times. Reload MC.

Also works when used in two separate sessions, so long as its on the same item.

Only mods were armourers, matter overdrive, and flan's, plus forge of course.

crash-2017-06-09_04.38.40-client.txt
crash-2017-06-09_04.44.40-client.txt

Edit for those seeking a solution: This is fixable by going into common.cfg and removing the duplicate entries.