The IPG says:
Once any of the above obligations has been fulfilled, or the trigger has been otherwise acknowledged, further problems are treated as a Game Play Error — Game Rule Violation.
Furthermore the Annotated IPG (
http://wiki.magicjudges.org/en/w/Annotated_IPG/Missed_Trigger) adds the following
“As convenient as it may be for judges to consider previously acknowledged but problematically-executed triggered abilities to be missed, they must nonetheless be treated as a different infraction.
For example, suppose Acetone attacks Neutrino with an unblockedArbor Elf equipped with Sword of Feast and Famine. During the combat damage step, Acetone untaps his lands but both players forget about Neutrino discarding. Even if this is noticed during the post combat main phase, this must be treated as a Game Rules Violation (GRV) by either rewinding the game or applying the appropriate partial fix, and not by simply asking Neutrino if she’d like the discard ability to be placed on the stack. It is also a GRV if you acknowledge a trigger at the proper time or earlier, and then, because of multiple things on the stack, you forget to resolve it.”
Once the triggers controller acknowledges the trigger as happening, it happened. Any errors that come after that, are handled by the appropriate infraction (probably GRV)
-bryan