Hi,
I have used the liquify effect on several test shots a while back, and have a lot of fun in doing so. I have handed some of the workload over the the team, and they have started working on our footage. They approahced me with a question this morning that I cannot answer, and was hoping I could run it by the forum members.
I do some keying work, and export a TIFF sequence that I hand over to the team. They import the footage and apply the liquify effect. The question came out, when they used the "Nudge" tool to make some changes. They were not satisfied with the effect, so "CTRL Z" was used to undo. When they hover the mouse over the footage and want to make another change, As soon as the mouse button is pressed, the footage reverts to the way it was before the undo command was issued.
Now, reading this statement, it sound s abit confusing to me, and I "know" what I am trying to say! 
It is basically that the undo command does not seem to work with the liquify tool. Is this a known issue (as I said, I have not tried this myself for some time) and I can tell them to work around it, or is this strange behaviour, and I should sit with them for a bit an dsee whats what? 
Thank you
Pierre