  During the analysis of the CR39 Nuclear Track Detectors (NTDs) of the SLIM
experiment exposed at the high altitude lab of Chacaltaya (Bolivia) we observed
a sequence of puzzling etch-pits. We made a detailed investigation of all the
CR39 and Makrofol detectors in the same stack and in all the stacks around the
candidate event. We found a second puzzling sequence of etch-pits (plus some
single etch-pits). The analysis of this configuration was important because we
were searching for rare particles (Magnetic Monopoles, Nuclearites, Q-balls) in
the cosmic radiation. Thus we analyzed in detail the evolution with increasing
etching time of the etch-pits. We concluded that the two sequences of the
etch-pits (and some other background etch-pits) may have originated from a rare
manufacture malfunctioning which involved 1 m^2 of produced CR39.
