Missing the Timing

Sometimes an optional effect can only be activated "when" a condition happens. In this case, you are only allowed to activate the effect when the activation condition being met was the last thing to happen in the game. If something else happened after the activation condition was met, then you have missed the timing, and cannot activate the optional effect. This mainly applies to Monster Cards with optional Trigger Effects such as "| Peten the Dark Clown" and "| Poison Draw Frog", but it can also affect Spell and Trap Cards such as "| Bottomless Trap Hole" and "| Heart of the Underdog".

Examples
For example, consider the following rulings: This is because you first Tribute "Peten the Dark Clown" as a cost, and afterwards the other monster is successfully Tribute Summoned. The last thing to happen was not "Peten the Dark Clown" being sent to the Graveyard, so it misses the timing.
 * ''If "| Peten the Dark Clown" is Tributed for a Tribute Summon, the last thing that happened was that a monster was Summoned, so the effect of "Peten" does not activate.

This is because you first Tribute "Peten the Dark Clown" as a cost, and afterwards "Mystik Wok" resolves and increases your Life Points. The last thing to happen was not "Peten the Dark Clown" being Tributed, so "Peten the Dark Clown" misses the timing.
 * If "Peten the Dark Clown" is Tributed for "| Mystik Wok" or "| Cannon Soldier", the last thing that happened was that those effects resolved, so the effect of "Peten" does not activate.

As explained, the last thing to occur is not drawing, so "Appropriate" misses the timing.
 * ''"| Appropriate" can only be activated if the last effect on the Chain ends with your opponent drawing a card(s) outside of their Draw Phase....
 * Your opponent cannot activate "Appropriate" after you activate "| Graceful Charity" since the last effect that resolves with "Graceful Charity" is discarding, not drawing, and your opponent misses the timing. But if your opponent already has an active "Appropriate" face-up on the field, he/she can draw 2 cards after you activate "Graceful Charity".

Versus New Activations
Activating new effects will not cause an effect to miss the timing. Even if you activate a new card, then it is still considered to be "When..." the activation condition is correct.

For example: Note that all three copies of "| Heart of the Underdog" will still activate. After you activate the first copy of "| Heart of the Underdog", then last thing to happen is you drawing a Normal Monster (you do not consider the activation of the first copy's effect), so you can activate the second copy.
 * Missing the Timing: Suppose you have 3 copies of "Heart of the Underdog" on the field, and draw a Normal Monster Card during your Draw Phase. All 3 copies of "Heart of the Underdog" immediately activate their effects, and because they activate simultaneously, they form a chain with Chain Links 1, 2, and 3. If you draw a Normal Monster Card for Chain Link 3 or Chain Link 2, your copies of "Heart of the Underdog" do not activate again. This is because they are "when... you can" optional Trigger Effects, and you "miss the timing" because you drew the Normal Monster Card during a chain and not as Chain Link 1. However, if you draw a Normal Monster Card for Chain Link 1, all 3 copies of "Heart of the Underdog" will activate their effects again.

"Bottomless Trap Hole" can still be activated because it is still considered to be "When "The Wicked Avatar" was successfully Summoned". It does not matter that the effect of "The Wicked Avatar" already activated as Chain Link 1.
 * When "| The Wicked Avatar" is Tribute Summoned, its effect begins a chain. You can chain "| Bottomless Trap Hole" or "| Torrential Tribute" to this effect.

Optional Effects that Do Not Miss the Timing
Note that this only applies to optional effect that are activated "When..." something happens. It does not apply to optional effect that are activated "If..." something happens, such as "| Watapon" or "| Ultimate Ancient Gear Golem", and it does not apply to optional effects are activated "After..." something happens (like "| Voltanis the Adjudicator"). Effects like these can never miss the timing.