Have a look at the following:
Several ethernet controllers in low-end products can only have a single unicast MAC address in their address
filter. These are: 2500 and 4500 which use the LANCE controller, and the 800, 1000, and 1600 which use the
QUICC controller (the 1700 is not affected). On these platforms only a single HSRP group is permitted, and
the interface address is changed to be the HSRP virtual MAC address when the group becomes Active. If
HSRP is being used on multiple interfaces of this type then each interface should be configured with a
different HSRP group number (See CSCdp18132)