Playing on Steam "next" version. Mods include Shrike's SM&M++ and a couple of extra stations.
I don't know how this happened, but my Manticore is only allowed to equip 7 items. Savefile:
https://drive.google.com/open?id=0By5RX1vUkuQ7UUpaOUFuU2FvWlU
I discovered this when trying to swap out the thermo cannon for an advanced tritium cannon; the game claimed I didn't have enough slots free to do that, despite the weapons both being single-slot items. Removing the thermo cannon still didn't work, and then I couldn't reinstall the thermo cannon (or install the tritium cannon either).
The ship had the following items equipped before this started, ordered in what I'm pretty sure is the order they were initially equipped in:
* Novaya Armor Repairer
* Smuggler's Hold
* Lithium Booster
* Thermo Cannon
* Thermo Shell Nanofac
* 500MW Reactor
* Pteracnium Megadrive
* Ares Launcher
I definitely didn't have any issues replacing my old Rasiermessier launcher with the Ares launcher, which was the most recent modification and happened maybe two systems ago.
Ok, the issue is that the Megadrive is a two-slot device in SM&M++. Somehow, you ended up with nine slots filled, which dropped to eight when you removed a gun. The game then blocked the re-install because you had full slots.
Which means the actual problems are twofold:
1: The interface isn't showing device slots in a way that can be tracked effectively.
and
2: Something let you install more equipment than you should have been able to. The only way I've ever done this is by updating the mod mid-way through a game...but the megadrive has been set up like this for a long time now.So that probably isn't it.
The former is a UI problem, the latter is probably a bug or a poorly-written dockscreen somwhere, and could be in vanilla or a mod.
I backtracked and found my old Titan fusion drive (I sold it at the Commonwealth station one station "back" from Point Juno), and replaced the Pteracnium Megadrive with that. That did indeed free up a slot, so I filled it with a random weapon, putting the ship at 8 full slots. The game was then happy to let me swap in the Pteracnium Megadrive for my old Titan, which would put me at 9 full slots.
So the problem seems to be that the dockscreen doesn't check slot limits properly when swapping drives. If I have no drive and 1 free device slot, then it won't let me install the Pteracnium Megadrive, so that's working properly.
It's probably worth checking if non-drive multi-slot devices behave similarly; I don't know enough about the debug commands to test that easily.
I'm glad that we were at least able to figure out how to reproduce the problem!
Fixed in 1.8 Beta 4. Thanks for reproducing this. The problem was indeed that the engine was not checking when replacing a 1-slot drive with a 2-slot drive. This would happen for all singleton devices (shields, launcher, reactor, etc.).