Some of the symptoms observed include:
-Differential signals may not have expected P/N crossing points. For example, with DIFF_SSTL15, it is expected thatP and N are to cross at 0.75V (Vcco/2), but simulations in Hyperlynx 8.1 show a much lower crossing point.
-Signals show unexpected Duty Cycle Distortion
-Differential signals terminated with a 100 ohm resistor might show unexpected ringing.