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, unless it is a certain type of optional Trigger Effect, such as ones phrased "If... you can...". Optional effects which can miss the timing are phrased "When... you can..."
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 "Appropriate" and "Heart of the Underdog".

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 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.