Electrical resistance tomography (ERT) has the capability to resolve the 3-D conductivity field inside a stirred mixing vessel using multiple planes of axially spaced sensors. This capability has already been exploited to qualitatively image vortex formation and geometry, pseudo-stationary gas-liquid mixing and unsteady dynamic brine tracer mixing inside a plant scale (1.5 m) stirred vessel (Mann et al, 1997(1)). This feature of qualitative imaging, arising from simplified reconstruction by back-projection, nevertheless can visualize key features of mixing characteristics without the need to resort to extensive iterations to converge on quantitative images, This approach is therefore useful in detecting and identifying pathological behaviour caused by equipment malfunction. Examples are presented for (i) misplaced gas sparger, (ii) inadvertent solids accumulation and (iii) displaced feed point behind a baffle.