Campaign Member

Campaign Member is one of those weird objects, like Contracts and Assets that have restrictions - and they will often surprise you with the little gotchas. 

Things you can't do:

• Have related lists on the page layout (you can relate Campaign Members to custom objects, but then you can't show the related list on Campaign Members). 
• Set up field history tracking - Big issue! https://success.salesforce.com/ideaView?id=08730000000BqXRAA0
• Like Opportunity Line Items, Assets, or Contact Roles; Campaign Members can't have Record Types. 
• Whist you CAN have Record Types on Campaign Members, and you CAN have different page layouts, The Campaign Member Type can only be controlled at the Campaign level and CAN NOT be updated via a Workflow or Process builder! STUPID!