In terms of debugging, there are certainly the goofs (off-by-one, = instead of ==, and so on) which are just lapses by the programmer. The danger I think is in losing the wisdom gained in having already solved the hard problem (of course it is helpful in this regard if the original programmers are still around somewhere). It's something akin to cultural knowledge. Don't underestimate what it's going to take to rebuild it.

But perhaps we can agree, then: rewriting code is a business decision, driven by the business needs of the company. Premature rewriting is just another form of premature optimization, and can get you in trouble by putting your resources in the wrong places. But when you have a tangible threat- when you are unable to adapt to accommodate needs that you are convinced are on your horizon, then you should do it.