Often it is necessary to replace several bricks in a wall, without rebuilding an entire section.
Sometimes this is because the bricks were flawed when they were made years ago, and have simply failed before the rest of the building. Other times they have had a harder life, due to moisture exposure or something of the sort.
Either way, these bricks need to be removed, and replaced with good bricks, before they compromise the rest of the wall. However, this process is often done poorly, causing greater problems than those already present. The most common offender is simply using the wrong brick. It may seem like the only problem with using the wrong brick is that it looks bad. But unfortunately that is not the case. Different types of masonry materials expand and contract at different rates in the cold. So what happens when you combine two materials is that they fight each other, and one always causes the other to crack, and fail. Other times, the bricks are poorly replaced without laying the bed of mortar full depth on all sides. This results in the replaced bricks not becoming part of the wall as they should. But instead, they act as plugs to keep water out, and no more.
As with all areas of restoration work, it is worth doing things right the first time.