Recent work shows how to use on-chip structures to measure the fabricated delays of fine-grained resources on modern FPGAs. We show that simultaneous measurement of multiple, disjoint paths will result in different measured delays from isolated configurations that measure a single path. On the Cyclone III, we show differences as large as +/-33ps on 2ns-long paths, even if the simultaneously configured logic is not active. This is over 20x the measurement precision used on these devices and over 50% of the observed delay spread in prior work. We characterize the magnitude of the impact of simultaneous measurements and identify strategies and cases that can reduce the difference. Furthermore, we provide a potential explanation for our observations in terms of self-heating and the configurable clock network architecture. These experiments point to phenomena that must be characterized to better formulate on-chip FPGA delay measurements and to properly interpret their results.