LociOiling Lv 1
Recipes can turn off undo with the command undo.SetUndo(false).
In the current version of Foldit, a recipe needs to issue undo.SetUndo(true) at the end to re-enable undo.
In "old Foldit", it appears that undo.SetUndo(true) wasn't required. Instead, undo was automatically re-enabled when a recipe ended.