Normal software only recovery methods cannot recover data that is overwritten once by any pattern, it takes a big budget and sophisticated techniques to recover data that has been overwritten only once. One overwrite is good enough unless you have the FBI NSA, NASA ect., wanting your data. But if your paranoid overwrite it 35 times, then disassemble the hard drive and grind the platters into fine dust, then scatter that dust in the open ocean over a 100 mile voyage, hopefully you wont get stranded on an island in the process, ;-)
Of course, modern operating systems can leave copies of " deleted" files scattered in unallocated sectors, temporary directories, swap files,remapped bad blocks, etc, but Gutmann believes that an overwritten sector can be recovered under examination by a sophisticated microscope and this claim has been accepted uncritically by numerous observers. I don't think these observers have followed up on the references in Gutmann's paper, however. So I can say that Gutmann doesn't cite anyone who claims to be reading the under-data in overwritten sectors, nor does he cite any articles suggesting that ordinary wipe-disk programs wouldn't be completely effective.