To reproduce:

BUGGY SCENARIO:
1. create new game objects ObjA and ObjB
2. make the following component, TestScript.cs:
using UnityEngine;

public class TestScript : MonoBehaviour
{
public GameObject obj;
}

3. Attach TestScript to ObjA
4. Attach a SphereCollider to ObjB
5. Drag ObjB into ObjA's public field
In the script, change the GameObject to a SphereCollider.

Result:
- Unity continues to think the reference is valid, showing "ObjB" in the
field.
- Various operations on the object/reference do not work as they should, as
the object is now corrupted.

To see the difference(correct behaviour):
1. create new game objects ObjA and ObjB
2. make the following component, TestScript.cs:
using UnityEngine;

public class TestScript : MonoBehaviour
{
public SphereCollider obj;
}

3. Attach TestScript to ObjA.
4. Attach a SphereCollider to ObjB.
5. Link click ObjA and drag ObjB into the public field.
6. Modify TestScript.cs and change SphereCollider to GameObject.

Result:
- Unity shows in the ObjA.obj field: "Type mismatch"