Normally a Switch port configured as a trunk port send and receive IEEE 801.q VLAN tagged Ethernet frames.
If a switch receives untagged Ethernet frames on its Trunk port, they are forwarded to the VLAN that is configured on the Switch as native VLAN. Both sides of the trunk link must be configured to be in same native VLAN.
Consider the below example. The trunk link connecting between the Switches, omnisecu.com.SW1 and omnisecu.com.SW2 are connected using a Hub. Some computers are also connected to the Hub. Hubs and computers are not aware of tagging protocols and the Ethernet frames originating from Hubs are untagged Ethernet frames.
It is not usual for a Switch port configured as a trunk port to receive untagged Ethernet frames. But it will happen if you are using Cisco VOIP phones in your network.