There not being a whole lot of mandatory "when" effects is probably why "if" vs "when" is brought up a lot. But yes, optional "when" effects are the only triggers that miss timing. Mandatory "when" effects and any "if" effects don't miss timing.
So suppose the following face-up monsters on the field are tributed as material for a Tribute summon. Since the event progression goes 1) monster on the field is tributed and leaves the field -> 2) Tribute monster is Tribute Summoned, we get:
- Reborn Tengu's "if" effect will not miss timing and can Special Summon another.
- Peten the Dark Clown's optional "when" effect will miss timing and cannot activate to Special Summon another.
- Tour Bus's mandatory "when" effect will not miss timing and will activate its effect.
Mandatory "ifs" and mandatory "whens" are functionally identical. Neither optional "if" effects, mandatory "if" effects, nor mandatory "when" effects miss timing but the latter 2 are not optional. And only optional "when" effects miss timing. There's another word that sometimes shows up, "each time" effects, but these don't miss timing either, like "if" effects.
The effects that don't miss timing don't have to have their triggering conditions meet as the last thing to happen when the chain resolves, they can meet their conditions higher in the chain. Example:
Player 1 controls a
Lord Gaia the Fierce Knight and Player 2 has a set
Call of the Haunted.
CL1: Player 1 activates
Mystical Space Typhoon, targetting Player 2's set Call of the Haunted.
CL2: Player 2 chains Call of the Haunted, targeting a
Blue-Eyes White Dragon in the Graveyard.
CL2: Call of the Haunted resolves, Special Summoning Blue-Eyes White Dragon from the Graveyard.
CL1: Mystical Space Typhoon resolves, destroying Call of the Haunted. Since Call of the Haunted was destroyed, Blue-Eyes White Dragon is also destroyed by its effect.
Chain finishes resolving. Now, Special Summoning the Blue-Eyes was definitely not the last thing to happen, but Lord Gaia the Fierce Knight can still activate its trigger effect here because its "if" condition has been met and "if" effects don't miss timing. Basically,
this page.
An example of an effect (rather than a cost) causing stuff to miss timing is
Goblindbergh's effect: "When this card is Normal Summoned: You can Special Summon 1 Level 4 or lower monster from your hand, also,
after that, change this card to Defense Position."
When you summon a monster with 1500 or more ATK with this effect, the opponent cannot activate
Bottomless Trap Hole since it reads: "
When your opponent Summons a monster(s) with 1500 or more ATK: Destroy that monster(s) with 1500 or more ATK, and if you do, banish it."
Since the last event to happen was not a monster being summoned but rather Goblindbergh changing to Defense position, Bottomless Traphole misses timing.
"Priority" is another of those old mechanics that don't exist anymore but the word still exists to describe something else entirely, so it can get misused by people.
