Why lfs couldn't use original host environment variables?

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http://unix.stackexchange.com – From the lfs doc: The exec env -i.../bin/bash command in the .bash_profile replaces the running shell with a new one with a completely empty environment, except for the HOME, TERM, and PS1 variables. This ensures that no unwanted and potentially hazardous environment variables from the host system leak into the build environment. The technique used here achieves the goal of ensuring a clean environment. What case will cause that problem? Is there any simple example? (HowTos)