Genome assembly is done through a hierarchical process, where first the contigs are assembled and then those contigs are connected together into scaffolds. Therefore, if an error is introduced in the assembly process at the contig stage, that error can propagate through the assembly chain.
Depth of sequencing is one possible source of assembly error. Assembly of de Bruijn graph builds contigs in the regions containing linear chain, but what if the linear chain is formed spuriously due to assembly error? Such errors can occur in low-coverage regions.