What are few Considerations about Salesforce Trigger?

upsert triggers fire both before and after insert or before and after update triggers as appropriate.
merge triggers fire both before and after delete triggers for the losing records and before update triggers for the winning record only.
Triggers that execute after a record has been undeleted only work with specific objects.
Field history is not recorded until the end of a trigger. If you query field history in a trigger, you will not see any history for the current transaction.
You can only use the webService keyword in a trigger when it is in a method defined as asynchronous; that is, when the method is defined with the @future keyword.
A trigger invoked by an insert, delete, or update of a recurring event or recurring task results in a runtime error when the trigger is called in bulk from the Force.com API.
Merge trigger doesn’t fire there own trigger instead they fire delete and update of loosing and winning records respectively.