When an arcane magician finishes a glyph they instinctively become aware of the cost of the glyph. If the cost of the glyph is present then the arcane magic automatically continues, if not the arcane magic fails. It is considered standard pratice that whenever peforming a glyph of a certian number of rings you first know a glyph with the same number of rings with a cost that cannot possibly be present in case the magic needs to be cancelled. The probelem being that if a magician stops a glyph without first converting it to this safe glyph the arcane magician may inadvertently cast a magic they did not know. This has issues for two reasons. For starters that when completed the arcane magician only becomes instinctively aware of the cost of the glyph and if the cost is present it is already too late to stop the magic. Unknown glyphs taking their casters right arm, life or loved ones an all too frequent occurance. Secondly is that while an arcane magician is instinctively informed of the cost of the magic, they don’t know what the magic actually does. An unknown glyph may take something of little cost, such as a few milliliters of water and in exchange detonate an explosion killing the caster. Casting unknown glyphs ending in more bad than good. The few that survive such an event may end up with a new, unknown glyph in their arsenal. Very few being insane or courageous enough (depending on the point of view) to become arcane explorers and attempt to discover multiple new glyphs in their lives.
Caster’s Perception.
Give a room of arcane magicians a glyph and ask them what the sacrifice associated with the glyph is and they will all the say the same thing. Send them to go get the sacrifice and often they will come back with different things. This phenomenon is known as caster’s perception. For example a sacrifice associated with a particular glyph may be a human heart. Depending on the arcane magician some will get the literal human organ (human attached or not), others will vow that when this particular human dies the cast may take their heart, others will sacrifice an object of a humans affections, others will sacrifice a humans ability to feel emotion. The list goes on. When asked the magician will always say the same thing. They sacrificed a human heart. How the sacrifice is interpretated varies from magician to magician. The real sacrifice being whatever the first thing a magician associates with the cost once they become instinctively aware of it. However this isn’t the limit of caster’s perception.
It is impossible to secretly sabotage an arcane magician. If when checking an arcane magicians sacrifice, if they supplied the human organ and you somehow managed to swap out the heart without the arcane magicians knowladge, they would still be able to activate the glyph. Provided a magician undeniably believes the sacrifice they have is a valid sacrifice it is. Arcane magicians can’t abuse this system themselves. Arcane magic being a measure of the soul and it being impossible to lie to ones own soul. This has two uses among those who aren’t arcane magicians however. The first as previously suggested is that provided one can convince an arcane magician that they can supply a suposedly expensive cast they can replace the materials with much cheaper alternatives and the magic still work. The second and significantly harder use is that if an individual can convince an arcane magician doesn’t have their required sacrifice, regardless of how real it may be, the individual can effectively seal away the arcane magicians glyph until such a time they develop a coping system. Considering that arcane magicians typically only comit a few glyphs to memory this can have devestating effects.