What will happen if two switch are connected to each other by two cables and spanning tree is disabled?

When a redundant connection is made, we will start receiving MAC address flapping notifications such as

%SW_MATM-4-MACFLAP_NOTIF: Host 003b.20c2.0264 in vlan 1 is flapping between
port Fa0/4 and port Fa0/5

These are triggered by the never-ending broadcast storm initiated by every layer two broadcast (such as an ARP request) from any host on the VLAN. Because STP is not present, neither switch can detect the switching loop. Ethernet frames have no concept of TTL, so they circle the loop endlessly.We can confirm the occurrence of Broadcast storm by checking the counters on the interface.