These are some event display images related to issue #1290. I have zoomed in to the collision vertex region.

Event #1, a good event, good vertex found at  (-0.00344922,0.0108172,8.942), primaries:

Event #2, a bad event, the good vertex found at (0.00449476,-0.00381626,-4.88609) and a bad vertex found at (-0.983723,-0.286838,34.5346), primary tracks in y vs. z (I had missed seeing the second vertex when looking in x vs. z; these tracks must lie mostly in one plane):

The TPC tracks pointing to the bad vertex at z=+34.5 can be seen clearly in this view. The FTPC tracks are doing something odd, as they seem to converge somewhere between the two vertices.

Event #2 (same event) globals:

It is a little difficult to see in this presentation, but the global tracks do look like they may converge on a real second vertex where one is found. Maybe it is a real secondary interaction?