[Lazy_Perfectionist]

What Burnsaber said.

Also: Make sure you have the proper mods on when starting a new singleplayer game. After that, the game automatically loads/unloads the appropriate mod when continuing.

Also: If you design a pretender for multiplayer, make sure mods are on. If the game detects an illegal pretender, you'll end up with something useless. Mostly, anything that might change a pretender chassis. Needn't worry about banners and hotfixes and stuff when creating a pretender, just the big stuff, like rebalance mods.