This was discovered after implementing the code in the previous post on, "Hopping from one Train Stop to the next, causes the old stop to reexecute its bindings", although I do not believe it is directly related:
This was most likely in the console log all along, but only was noticed because of the fact that one of the executables, which was released after visiting a new tab, was being held onto and was trying to get reused. But since it had been released, it could no longer be used, and therefore threw up an exception, thus bringing the !ERROR! to our attention.
Reset the ChangeEventPolicy of the offending iterator back to the default, rather than "ppr".
According to the docs, ChangeEventPolicy can be used to perform automatic refresh of any UI component bound to the iterator when data is changed in the model and the docs actually recommend using it when there are a lot of components on a form that need ppr, however this was not needed in this particular case (and so far, nowhere else in our app).