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We present results from two \sim30 ks Chandra observations of the hotatmospheres of the merging galaxy groups centered around NGC 7618 and UGC12491. Our images show the presence of arc-like sloshing cold fronts wrappedaround each group center and \sim100 kpc long spiral tails in both groups. Mostinterestingly, the cold fronts are highly distorted in both groups, exhibiting'wings' along the fronts. These features resemble the structures predicted fromnon-viscous hydrodynamic simulations of gas sloshing, where Kelvin-Helmholtzinstabilities (KHIs) distort the cold fronts. This is in contrast to thestructure seen in many other sloshing and merger cold fronts, which are smoothand featureless at the current observational resolution. Both magnetic fieldsand viscosity have been invoked to explain the absence of KHIs in these smoothcold fronts, but the NGC 7618/UGC 12491 pair are two in a growing number ofboth sloshing and merger cold fronts that appear distorted. Magnetic fieldsand/or viscosity may be able to suppress the growth of KHIs at the cold frontsin some clusters and groups, but clearly not in all. We propose that thepresence or absence of KHI-distortions in cold fronts can be used as a measureof the effective viscosity and/or magnetic field strengths in the ICM.

Irregular sloshing cold fronts in the nearby merging groups NGC 7618 and UGC 12491: evidence for Kelvin-Helmholtz instabilities