To claim unemployment benefits or transition smoothly to a new job, Japanese workers need a separation notice (rishokuhyo) and a tax withholding slip (gensen choshuhyo). Bitter companies will simply "forget" to process or mail these, forcing the former employee to beg for them or drag the labor standards office into the fray.

While withholding a final paycheck is strictly illegal under the Labor Standards Act, companies might delay it or refuse to pay out expected severance (taishokukin).

If the industry is small enough, word travels fast and people can be blacklisted.

And just like many USians believe you have to give two-weeks notice, many Japanese believe you must go through that process to leave a company (with possible good reason considering how hard the companies can make it).

From what I've heard, it can affect your future job prospects, yes. I've heard of some companies in Japan that would try to find out which company you're trying to leave them for in order to lower the likelihood you'd get hired there if you don't leave them "the right way". But I don't have any personal experience in this and I haven't talked about it much with the people I know in Japan, so this is just hearsay you should take with a grain of salt.