Your opponents can't look at the card they own that you manifested.

If a creature you control but don't own dies at the same time that your life total becomes 0 or less, you lose the game before Thieving Amalgam's last ability can save you.

If a creature you control but don't own dies at the same time that its owner leaves the game, Thieving Amalgam's last ability triggers. No player loses 2 life, but you gain 2 life.

In the Commander variant, you can produce mana that isn't of your commander's color identity if an effect lets you produce mana of that color or mana of any color, even though those symbols can't appear on cards in your deck. You may be able to turn a manifested creature card face up this way if it's not the same colors as your commander.

In a multiplayer game, if a player leaves the game, all cards that player owns leave as well. If you leave the game, the creatures you manifested with Thieving Amalgam's triggered ability are exiled.