Using a crafting table on 3rd party servers, the crafting inventory is automatically sent when you touch a crafting table, which can cause some issues, such as crafting while crouching. On vanilla, you aren't able to craft while crouching, but on a 3rd party server, you crouch and try to craft and the inventory is sent and won't craft anything. The 3rd party server can do nothing about this, since the client handles this.