I enjoy a good rules debate.
I enjoy comparing what the authors most likely intended with what they actually wrote on the page.
Essentially, I enjoy pointing out issues that most likely need an errata.
One such instance I've come across recently involves the Space Marine Chapter Master. Chapter Masters have an option to take items from the Special Issue Wargear List. They can take a Bike. The Bike has two restrictions. The first is that it is mutually exclusive with a Jump Pack. The second is that it can't be selected my models in Terminator Armour. The Chapter Master has a second, later option to choose Terminator Armour. The Armour has no restrictions.
1. Choose Chapter Master to fill an HQ slot.
2. Give Chapter Master a Bike. He doesn't have a Jump Pack and he doesn't have Terminator Armour at the time of the option selection.
3. Give Chapter Master Terminator Armour. There are no restrictions here.
4. You now have a Bike riding Terminator.
Now, we have a couple of things going on here. We know that options are selected sequentially and that the next option can't be selected before the first option is resolved. We know this from the "Biker Special Weapon" issue. Bikers have an option to take items from the Special Weapons list, but they don't have a Melee weapon to trade in. GW added an errata allowing them the option to swap Bolt Pistol for Chainsword. This option has to fully resolve for the Biker to choose a Special Weapon. Hence, options fully resolve and resolve sequentially.
The second thing going on is that the codex places a restriction on Terminators that they can't select Bikes. There is no restriction on Bikers saying they can't take Terminator Armour. This is kind of like how O'Vesa (a Monstrous Creature Independent Character from the Farsight Enclaves Supplement) can join another, non-MC IC... but no other IC can join him since he's an MC and IC's can't join MCs. The action is allowed in only one direction. Bikers can put on Terminator Armour, but Terminators can't hop onto Bikes.
I'm certain GW didn't intend this, but it's there in black and white.
I don't think I'd ever do this, and I wouldn't allow it during an organized play event such as a tournament, but there are plenty of fluff reasons this could happen...
Is it that unreasonable to believe that your Chapter Master is riding a Chapter Relic Bike? An up-armoured beast with a built-in forcefield and the ability to teleport onto the field? Granted, the 5++ is worse than his 4++ and he'd lose the ability to Sweeping Advance, but it's a valid fluff reason.
I don't know... just some thoughts. GW really needs to do some community play testing or hire a technical editor.