Weird error msg

[Ling]

I have this form which is quite long and it has a subform
Weird thing is , when I click this particular combo box
(cmbSize), and I try to scroll the subform, I get an error
saying 'Could not update; Currently locked by another
session on this machine'.

However, when I click on other combo boxes that would also
affect the calculations of fields in the subform, this
error msg doesnt show up. What is wrong?

THanx..

Ling