No longer can we view a PC trace as a perfect wire. Any trace longer than a few inches will look like a transmission line. Minimizing noise is crucial in every fast design. An emulator only makes the noise problems worse. Electrically, especially since we're now talking transmission lines and not wire, this configuration little resembles an NMOS or CMOS CPU's output.
Multiplexed busses cause the biggest problems.
The 80186, for example, outputs an address and then the data on shared wires. When the address is valid an ALE signal is asserted, so the target system logic can latch it. Some power will flow between the emulator and target, even if only in the signal lines. If the target's power distribution tracks on the PC board are poorly designed, then the emulator can induce even more noise. You would be amazed by the number of systems we see with miserable power and ground tracks.