Solemn Warning

Solemn Warning:

Solemn Warning can ONLY be activated if a card is activated that Special Summons something. And by activating the card, I mean placing it Face-Up on the field, like Monster Reborn, or Call of the Haunted, or Gorz, Emissary of Darkness.
There are some cards however, that Special Summon when you activate their effects. Meaning, cards like Fusion Gate, Valhalla, Hall of the Fallen or Infernity Launcher.
This is NOT card activation, this is effect activation. Solemn Warning's text does say it can negate the activation of an effect activation, but this refers to monster effects.
So, since Warning can only negate the activation of a Spell/Trap CARD, it cannot negate the activation of a Spell or Trap Effect, like Fusion Gate or Infernity Launcher.

Also, what people do wrong a lot, is using Solemn Warning on the Summon of a Fusion or Ritual Monster. This is not possible, unless the Fusion was done by Contact Fusion.
To negate ritual or Fusion Summons, you have to negate the card activation that Summons the monster. For example Polymerization, or Gishki Aqua Mirror. Warning cannot negate the Summon of a Monster that was Summoned by Polymerization or Gishki Aqua Mirror, only the activation of the card. This is, because when it is Summoned, a card effect is resolving, and basic Game Rules state, you cannot, under any circumstances, start a chain while another chain is resolving.