Learn English – How else can I express the concept of ‘ just to be on the safe side’

Holub employs a mixed metaphor (as Robusto suggested) and the descriptions of the book allude to at least one other expression, which is "just enough to be dangerous" (as in "I/You/They know just enough to be dangerous.").

As with most metaphors, they are designed to evoke a mental image of equivalence. In this case, Holub appears to be addressing some particular aspects of computer programming where he believes that programmers are knowledgeable enough (enough rope) to be dangerous (shoot foot) and therefore they are "a danger" to the programming world.

Using a mixed metaphor is often done for humorous effect, which appears to be Holub's intent here. As this review states "Holub manages to make a serious subject refreshingly readable by sprinkling the text with humor and insight."

If a mixed metaphor like this were to have a commonly used substitute, the substitue would probably lose the full effect of the metaphor. But you might say simply use "Programmers know just enough to be dangerous" (which isn't idiomatic, but requires context), which I already mentioned. Stated alone, it doesn't evoke the same image as a hanging and an unintended discharge of a weapon.

You can use the expression "You can do it of your own free will" or "You can do it of your own volition", or simply "It is voluntary", in other words, "It is not compulsory", to describe things that are volitional.