Sometimes, the old code itself may contain bugs that never surfaced on the mainframes and minicomputers on which it was developed and tested. If we convert it or rewrite the old code, the converted code may still contain that bug, and a modern compiler or a new application can make that bug come alive. If that happens, it can take quite a bit of work to assign or shift blame, and it is likely that the people who did the conversion will be blamed, because “nobody had seen such errors for 40 years in that software”.
An example of this was discussed in a comp.lang.fortran thread three years ago. Eispack was published in the mid 1970s, and the bugs were observed in 2018 by Jack Wolfman.