I've also been hitting the segdef problem more lately.
Main cause on my side is very large forms from the client side.
I've been redesigning the forms with different tabs, calling separate procedures.
However, I found that went I call a new procedure from within a NWF-tab, the controls in the new NWF seems to be broken somehow.
For example, the checkboxes break completely in the second NWF, whilst the checkboxes on the calling NWF still works.
I thought it had to do with some faulty control and/or code, but after redoing the NWF, I ran into the same issue.