5. Anything that is very frozen
And again, if you have a strong blender, this is allowed, however, if your blender is of standard strength, harder and solid things could cause problems to the device. Ice cubes could be problematic for crushing, frozen fruits can remain in pieces, while blades could be damaged under the pressure.
Instead, if you are trying to make a smoothie in the standard blender, it would be desirable to let the fruit dissolve for about five to ten minutes before inserting it into the blender, and it is also recommended to use smashed ice in the blender.