If you mean psilocybin mushrooms, then I'll go against the typical harm reduction narrative and say that taking
too little is likely to result in a worse experience than taking
too much. You can't physically eat enough of the mushroom for it to become a medical concern, unless there's dangerous drug interactions happening or you have a preexisting heart condition; worst thing that can happen to you is gastrointestinal upset and a bad trip.
Now a word about interactions... anything with a MAOI effect can lead to serotonine syndrome; grapefruit (juice, fruit, etc.) can lead to the same thing. (Don't ask me how I know lol... xD)
With too little you are risking being stuck in a highly unpleasant inbetween, trapped in your present moment with a greatly and unpleasantly amplified sensorium, and worst case you'll end up cleaning your apartment while feeling like all the demons of hell are on your case and quoting your every sin at you.
With too much, you're going to just "go away." It can be pleasant or unpleasant, worst case you might be risking a perceptual eternity in limbo, but if there's no "you" present as such (and with too much there won't be) it'll be somewhat easier to handle.