This was an interesting one, I was trying to download a IOS image to a switch but the connection was being denied.
It turned out the connection was coming from a VLAN interface on the switch that did not have access to the TFTP server.
You can force the VLAN interface used with this command:
ip tftp source-interface vlan 5
Where the VLAN 5 is the interface from which the want the traffic to emerge.