Pressing C doesn't cancel out of the Upgrade Ship dialogue, only pressing X does, and only near that specific NPC. You can still move during it, and if you move away, you can never get rid of it until you go next to that NPC again and press X, which could be confusing. If you then go to the shop, the Upgrade Ship dialogue is still covering everything else, including the Shop dialogue. I'm not sure this can be considered game breaking, but it's one I noticed right away.