About merging vertex edges of different materials namely plane and cylinder I wonder if anyone could help me with one issue I have run into while using Blender and modelling my character. 
Actually Blender is not merging vertices of hand of my character with wrist properly under subdivision modifier applied at 2 subdivisions in the viewport. when I merge them vertex at merging points distorts and when I merge vertex of plane everything goes fine but when I try to model the hand with a cylinder and merge the hand with the character (a plane), vertices at the merging points distort. I don't know because of what kind of tension or something. editing.
I would be grateful if you could tell me how to solve this problem.
Select all and press Shift N (recalculate Normals, outside).