Most likely this is occuring because when you copy out of JetForm, it is being stored in the clipboard as some data-type other than text. When you paste into Word (or Notepad) and then copy again, it gets copied into the clipboard as a different data-type, one which InfoPath understands.

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Greg Collins [InfoPath MVP]
Visit http://www.InfoPathDev.com
I have a situation where a client wants to copy their saved data from a
JetForm Filler form and paste it into the same, converted, InfoPath form. I
tried this and it does not work. I did find that pasting the data into a
Word document and then copying it again will then allow you to paste into an
InfoPath form field. You would think that you could copy and paste between
two 32-bit apps.
Any ideas on why this is happening?

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Thanks! -Steve M.