Before you embark on this sort of project, make sure you have all the permits etc. that you might need for the work. This is important if the building is very old as you may need to check with your local historical committee for any restrictions as to the type of work you can do on it.
Older homes were made with durable, solid materials. When doing repair work, make sure you consider these materials and try not to replace them with less durable ones like plywood or plastic. Areas that often need repairing or even replacing are windows and doors. You could ruin the look of the home if you replace these with modern options.
Modern materials and even techniques could have a negative impact on the original construction materials of the home. For example, if you try waterproofing old brick and masonry, you could actually cause it to deteriorate. This could happen with fire retardants and insulation also.
Other issues that will likely come up with an old home are toxic substances found in old building materials such as asbestos and lead. This is an area where you should call in professionals to take care of them and remove them safely.
In review, while this is only a few of the potential mistakes that can be made with a retrofit, the point is to carefully research the best way of fixing up this kind of property, without losing its old charm and character.