If a mistake is made in a Pokemon game, and neither player catches it, does the player who failed to do the correct action lose the game, or is the game a draw, or do you continue to play the game since the mistake was overlook. Example: A coin was suppose to be toss and if tails the Pokemon could not attack. I failed to (overlook) the toss, several turns later I realize my mistake. This mistake may have put me ahead in the game (if the toss had been tails.) I forfeited the game and later try to look up an official rule on this to no avail.
It depends whether you are playing casually or in an official tournament. For tournaments, it also depends on its type - beginner-friendly Tier 1 tournaments like League Challenges are different to Tier 2 tournaments where players are expected to abide by the rules all the time.
In casual play, both players have to agree on the resolution of the game state, i.e.
- ignore the mistake
- try to rewind the game state to the point before the mistake
- the opposing player draws a Prize card (or more of them)
- player who made the mistake loses
In tournaments, everything depends on the official judge (or more of them) - they are the only ones who can rule the resolution of the game state. In addition to possible resolutions above, they can also give a Caution or a Warning to one or both players.
The main document for judges is Pokémon TCG Penalty Guidelines, available from Rules & Resources page of official website.
A coin was suppose to be toss and if tails the Pokemon could not attack. I failed to (overlook) the toss, several turns later I realize my mistake.
Using Penalty Guidelines from October 2019, the actual resolution depends on "rewindability" and overall damage of the game state. Because several turns have passed, the problem would probably fall into 8.1.2. Gameplay Error - Major, but might also be closer to 8.1.3. Gameplay Error - Severe.
Both players are responsible for maintaining the game state. If the game state is irreversibly broken, the opponent might get a penalty for 8.1. Gameplay Error as well.
The actual penalties strongly depend on the particular situation.
According to the Play! Pokemon Penalty Guidelines it depends on the severity of the mistake made, and the Tier in which you are playing. It can range from a Caution all the way up to a Game Loss.
For you example it would probably be a Game-Play Error - Major for "Using an Ability when a card prevents its use". For this error in a Tier 1 match you would get a Warning, in a Tier 2 your opponent has to take two fewer prize cards. In either case your opponent would get a Caution for failing to maintain game state.