Population studies of Kepler's multi-planet systems have revealed a great deal of structure in their underlying architectures. These population models can now be used to make predictions about the presence and properties of additional planets in systems with known transiting planets. Here, I will describe how we use a statistical model for the distribution of planetary systems to compute the conditional occurrence of planets given a Kepler-detectable planet. While unseen planets may potentially be discovered by radial velocity (RV) follow-up observations, they can also add a source of systematic error in efforts to fit the semi-amplitude (K) of the transiting planet. I will show that attempts to measure the K of the transiting planet when there are an unknown number of planets often requires significantly more observations than in the ideal case (when there are no additional planets). Planets around 10 day periods, common among the TESS planet candidates, with K comparable to the single-measurement RV precision typically require ~100 observations to measure their K to within 20% error. These results highlight an important and previously unaccounted for source of error in measuring the masses of transiting planets with RV follow-up, which should be considered when planning RV surveys.