New unusual types of extended defects resembling bunches of divergent dislocations are observed in high quality synthetic HPHT single crystal diamonds grown at the lowest possible PT-conditions. Thorough characterisation of the samples by X-ray topotomography, optical microscopy and vibrational spectroscopy allowed the conclusion that these defects are related to dislocations decorated by oxide inclusions. An unusual feature of the defects is their origin: small-angle grain boundaries formed during growth instabilities. Although similar defects were observed in some isostructural materials, their presence in diamonds was not reported earlier.