HaCaT keratinocytes are described as "spontaneously immortalized "(see paper below) due to aneuploidy or chromosomal alteration. From the paper I get that this is a literal term, that the cells just continued growing (?) in vitro and never went into senescence without any "human-induced" transformation. Is this a common procedure for cell immortalization or a rather rare example?
It depends how you define it. Many early cell lines (as well as some modern ones) were derived from tumors. Tumors are spontaneously immortalized cells which became immortalized in vivo. If you're asking how often researchers collect primary cells (non immortalized) and grow them with the hopes they will become immortal the answer is pretty much never.