Diablo 3? If you hope to keep having fun, you might want to avoid the game-breaking bug discovered by fellow players. According to many posts on the Blizzard forums, equipping a Templar follower with a different shield may result in the game desynchronizing from Blizzard's servers. Users are also reporting that they are unable to log back into Diablo 3 – specifically encountering timeout error 3006 – even after restarting the game.
As noted by Eurogamer, the issue may be restricted to the Demon Hunter class. Blizzard is currently looking into the problem. In the meantime, regardless of your class, you should probably avoid swapping equipment with the Templar follower.
Update: Apparently the issue arises specifically if you swap items that you currently have equipped with the Templar. Still, probably safer to not mess with a Templar's equipment at all for now..