After you set a certain static MAC address to a virtual machine adapter you cannot power on the virtual machine.
In the vSphere Web Client, after you assign a MAC address within the range
00:50:56:7F:YY:ZZ to a virtual machine, attempts to power the virtual machine on fail with a status message that the MAC address is in conflict.
00:50:56:XX:YY:ZZ is not a valid static Ethernet address. It conflicts with VMware reserved MACs for other usage.
You attempt to assign a MAC address which starts with the VMware OUI
00:50:56 and is within the address range allocated for host VMkernel adapters on the vCenter Server system.
If you want to preserve the VMware OUI prefix, set a static MAC address within the range
00:50:56:3F:FF:FF. Otherwise, set an arbitrary MAC address whose prefix is different from the VMware OUI one. For information about the ranges available for static MAC addresses that have the VMware OUI prefix, see the vSphere Networking documentation.