I know some guys who worked in IT at a major UK Company - they were installing a completely new system and it took forever - every weekend they took the system down, installed the new bits and brought it to life on Sunday evening only to hear around 11:00 am on Monday that yet another hitherto unknown, but largely used and financially significant, application wasn't working
there was stuff there that had been patched in from the mid 60's apparently that people were still happy (indeed insistent) they used for everyday work
scarey stuff