I usually find architecture problems to be the hardest to solve. The hardest one I've had to deal with is taking a legacy web application ~3 million lines of code and giving it some form of architecture so the product can have a sustainable future. Some of the issues included inline CSS styles, Core logic written in linear Classic ASP, ASP Web Forms written in a linear fashion and so on. As you can guess, what made it hard was trying to solve these issues without breaking anything, this is an obvious example of why automated testing and code quality are so important.
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I usually find architecture problems to be the hardest to solve. The hardest one I've had to deal with is taking a legacy web application ~3 million lines of code and giving it some form of architecture so the product can have a sustainable future. Some of the issues included inline CSS styles, Core logic written in linear Classic ASP, ASP Web Forms written in a linear fashion and so on. As you can guess, what made it hard was trying to solve these issues without breaking anything, this is an obvious example of why automated testing and code quality are so important.