This has just happened and I'm stumped as to why:

Opened a workbook on my machine. Each sheet has 3 buttons, each assigned to
a specific routine. Generally button1 assigned to General_Button1_Click etc.

I checked that this was the case, then saved the workbook to my C drive. I
then copied it to a CD-Rom.
On a Colleague's machine, I copied it to a folder on their C drive.

I opened the workbook. When I clicked a button, a message box declared that
the file could not be found. I right-clicked the button and selected "assign
macro".

The assignment box did not show General_Button1_Click
it showed
F:\Add-Ins\MasterEstimateBofQCode.xla!_
General_Button1_Click

I checked the original on my machine and that seems to just have
General_Button1_Click in the assignment box.

Confused.

Regards.