What happens if I probe somewhere in the middle of differential traces? (N1021B differential TDR probe)

Imagine how TDR pulses travel through the probe into the device under test: At the probe tip, the TDR pulses split into two parts and travel in opposing directions away from the probe. In other words, the probe sees two impedances that are connected in parallel. The net effects of probing in the middle of traces are (1) you will greatly attenuate the transmitted signal, (2) you will cause a big discontinuity in the line that will send a reflection back to the source, and (3) the signal viewed using the TDR probe will be very attenuated versus the intended signal. In addition to the energy loss from the reflection mentioned above, the probe "sees" two impedances that are connected in parallel. If you want to view a signal on a transmission line with minimal loading, use the InfiniiMax high frequency differential probes.