Is there any real case when a switch after it receives a unicast frame on one of its ports would flood the frame on all its ports except the port it received the frame on?
I see this being possible only if the destination MAC address was already flushed by the switch from its MAC table when the unicast frame arrived, whereas that destination MAC address is still valid in the host's ARP table (not needing os to send a broadcast ARP for learing that MAC address), but I don't know if this could happen in real cases.
What is the default timer for flusing an entry from the MAC table of a switch ?
Apologies. Yes if an end device has the mac-address of the destination device but the switch does not then it would flood the frame out all ports (except the one it received on).
It could also occur when the mac-address table is full and the switch can no longer store any entries. Have a look at this link which covers unicast flooding within a switched network -