This phenomenon is observed in p-block elements. Elements like tin, gallium, indium, thallium, germanium, lead, tellurium, animony and polonium show additional oxidation states besides their own group number oxidation states. Elements with an outer electronic configuration ns2npx can either remove all s and p electrons or they may remove only x number of electrons from their outer electronic sub-level. This is due to the extra stability of the s-level electrons which remain intact with the atom. Thus, a molecule like tin of lead show two different oxidation states- (1) their usual oxidation state[OX]+ and another [OX - 2]+. The later is due to the inert pair effect of inert s-orbital electrons. Tin shows Sn+4 state which is its group number oxidation state, while it also shows Sn+2 which is due to its inert pair effect.
The lower oxidation states in these elements are comparatively more stable than the higher group number oxidation states due to extra stability of the s-electrons. This is highly evident in case of heavier metals.