Simple collision function acting strangely

On collision, score increases by 10. I used Debug.Log in the collision function first to make sure everything worked and it worked perfectly. Now the score starts at 20, and on collision one third of the time adds 20, one third of the time subtracts 20, and the other third of the time it does not change. I must be making a mistake in this code somewhere because like I said, I used debug to check that my objects were set up right. I appreciate any advice.

 public int score = 0;
 
 void OnCollisionEnter (Collision col)
 {
     if(col.gameObject.name == "Plane")
     {
 //On collision, add 10 to score, access text, change text to score. 
 score += 10;
     TextMesh ScoreMesh = GameObject.Find("Score").GetComponent<TextMesh>();
         ScoreMesh.text = score.ToString ();
     }
 }