Many cards and effects can only be activated under certain conditions, or can only be used against certain targets. But because other cards and effects can be chained to them, and resolve before the original card resolves, those conditions aren't always still correct at the end of the chain.
In general, if a card has specific conditions in order to be activated, those conditions only have to be correct at the time the card is activated. If they are no longer correct when the card resolves, the card's effect still resolves.
However, if a card has specific conditions regarding its target, and those conditions are no longer correct at the time the card is activated and when the card resolves, then the card's effect disappears.
Examples of cards with specific conditions for activation, and how those conditions no longer have to be correct at resolution:
Types of Monster Cards
Types of Spell Cards
Types of Trap Cards
Other Types of Card
ATK - DEF - Level - Attribute - Type
Playing Field Zones
Types of Summon
Types of Monster Effects
Types of Effect Monsters
Types of Spell Speed
Aqua - Beast - Beast-Warrior - Dinosaur