Noticed on OS X 10.9, confirmed on OS X 10.4. This happens no matter if "Send system keys" is on or off. xev on the server side doesn't see the Insert key press or the following mouse click at all.
Pressing 'Insert' on the OS X desktop (10.4, 10.9) brings up the same '?' mouse pointer.
When pressing 'Insert', this shows up in tlclient.log: 2013-11-04T14:26:26: vncviewer[E]: 2013-11-04T14:26:26: vncviewer[E]: Mon Nov 4 14:26:26 2013 2013-11-04T14:26:26: vncviewer[E]: Viewport: Unexpected release of FLTK key code 65384 (0xff68)
OS X does not have any "Insert" key at all. In our lab we have a PC keyboard connected to the OS X machine, this is likely the cause of the confusion. So since there's no Insert key, I don't see that our client does anything wrong...
That key is called "Help" on macOS, so we must be triggering something along those lines. Also note that Apple has abandoned this key and modern keyboards ship with a Fn key in its place. So this bug might no longer be relevant.
Wikipedia claims the key got deprecated way back in 2007: https://en.wikipedia.org/wiki/Help_key It also doesn't seem to have any effect any more. I tried on macOS 13, and nothing happened when I press the key. It still works in the ThinLinc session, though, where I get XK_Help. I don't think older versions of macOS is anything we need to worry about.