1. Hydrogen resembles alkali metals as well as halogen so no fixed position could be given to hydrogen in the periodic table
2. At some places metal of higher atomic mass is placed before an element of lower atomic mass e.g.cobalt (58.93) is placed before nickel (58.71).
3. Some elements placed in the same sub group differ in the properties e.g.Manganese (Mn) is placed with halogens that totally differ in their properties.