I have often encountered code littered with lots of nested if and else statements. I consider it a serious code smell. Deeply nested code, especially if the nesting is due to conditionals, is very difficult to follow, wrap your head around and test. It even makes it difficult for the compiler or runtime to optimize. In many cases it is possible to flatten deeply nested code by very simple means. Here are a few examples in Python, but the concepts translate to most languages.
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