We report on a timing analysis of archival observations of the anomalous X-ray pulsar 1RXS J170849.0-400910 made with the RXTE Proportional Counter Array. We detect a new large glitch (Δν/ν 3 × 10-6) that occurred between 2001 March 27 and May 6, with an associated large increase in the spin-down rate (Δ/ 0.3). The short time (1.5 yr) elapsed from the previously detected glitch and the large amplitude of the new spin-up place this source among the most frequent glitchers, with large average glitch amplitudes, similar to those of the Vela pulsar. The source shows different recoveries after the glitches: in the first one it is well described by a long-term linear trend similar to those seen in Vela-like glitches; in the second case the recovery is considerably faster and is better described by an exponential plus a fractional change in the long-term spin-down rate of the order of 1%. No recovery of the latter is detected, but additional observations are necessary to confirm this result. We find minor but significant changes in the average pulse profile after both glitches. No bursts were detected in any light curve, but our search was limited in sensitivity with respect to short (t < 60 ms) bursts. Observed glitch properties are compared to those of radio pulsar glitches; current models are discussed in light of our results. It appears that glitches may represent yet another peculiarity of anomalous X-ray pulsars. Starquake-based models appear to be preferred on qualitative grounds. Alternative models can be applied to individual glitches but fail in explaining both. Thus the two events may as well arise from two different mechanisms.