I just ran a test in VFP7 SP1 with textbox on a form and it works the same: with AllowTabs = .T. tab characters doesn't trigger the InteractiveChange event. I think, you can use Keypress event (it fires before IE) in addition to InteractiveChange event to get around it.
Anyway, I think it's a bug. You can report it here http://support.microsoft.com/default.aspx?scid=%2Fsupport%2Fvfoxpro%2Freport%2Freport%2Easp

>Using VFP6 SP4, I've run into what appears to be a VFP bug. I've got an editbox with AllowTabs = .T., i.e. permitting the user to type tab characters into the control. Although this does indeed allow the entry of tab characters, it fails to trigger the InteractiveChange event, unlike the behavior for other characters.
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>A quick search on UT for the past 2 years did not show any reports of such problems. I wonder if anyone can confirm the existence of this bug, and whether it has been fixed in VFP6 SP5 or VFP7. Thanks in advance for any further info.
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>Mike