We investigate in detail two anomalies in metal-semiconductor field-effect transistors (MESFET) which are detrimental to their operation in a GaAs integrated circuit (IC). The first of these is side-gating and consists in a parasitic very-low-frequency transconductance between a MESFET and an isolated electrode or side gate sharing the same semi-insulating (SI) substrate. The second anomaly is kinetic and consists in a parasitic transient in the drain-source current following an abrupt change in drain-source voltage. The time constants of the transients are observed to be very sensitive to the side-gate electrode potential. The effects of different technological and operational parameters are studied. One surprising result is the very high sensitivity of side-gating to chemical wet and dry treatments and to dielectric encapsulation of the unprotected substrate surfaces. We also report low-frequency oscillations related to side-gating. Physical models are proposed and yield consistent interpretations of our experimental observations.