What is a flag-checker? Something about chess/checker game?
Code that makes sure the player's taken a specific action before accessing a certain portion of the game.
To elaborate, developers often use "flags" as a way to check if certain conditions are met.
You might have seen the terms "route flags" or "
event flags" used when discussing visual novels or dating sims. Imagine a game where in Act 1, you meet all the possible romance interests and have the chance to interact with them. In Act 2, the game might put you on a specific route depending on what flags you've "raised" (e.g. which guys/girls you talked with, what events you triggered, the choices you made, etc.)
This term is also used outside games, in the
general programming sense. You might toggle command-line flags in your terminal, or a
startup might use flags to decide whether certain features are enabled for some users. The term "flag" comes from the nautical/military context, where a boat might have a certain flag up to
signal certain things to other boats.
In the specific case of Anverse, one example could be that the girl could have different dialog depending on what actions the player took on the way. (e.g. whether he took the mountain pass, or the valley, or saved a person in distress). The developers might not have accounted for the possibility where a player would not have taken any of the actions, and therefore no condition can be met, causing a crash.