I've just tested and if you have more than 20 recipes registered for same ingredients(and quantity) combination, client will suffer a crash to desktop on combine or on looking at one of the items with the other in hand.
Most common recipe is Nails + Planks so if a server has several mods which register recipes of these types then players can crash on combine.
Unsure what the solution could be here but i think the issue is due to too many tooltips to scroll through.