  Observations of accreting black holes often provoke suggestions that their
jets precess. The precession is usually supposed to result from a combination
of the Lense-Thirring effect and accretion disc viscosity. We show that this is
unlikely for any type of black hole system, as the disc generally has too
little angular momentum compared with a spinning hole to cause any significant
movement of the jet direction across the sky on short timescales. Uncorrelated
accretion events, as in the chaotic accretion picture of active galactic
nuclei, change AGN jet directions only on timescales \gtrsim 10^7 yr. In this
picture AGN jet directions are stable on shorter timescales, but uncorrelated
with any structure of the host galaxy, as observed. We argue that observations
of black-hole jets precessing on timescales short compared to the accretion
time would be a strong indication that the accretion disc, and not the standard
Blandford-Znajek mechanism, is responsible for driving the jet. This would be
particularly convincing in a tidal disruption event. We suggest that additional
disc physics is needed to explain any jet precession on timescales short
compared with the accretion time. Possibilities include the radiation warping
instability, or disc tearing.
