Indentation not Tabulation People(I) make mistakes often because they mixing the idea of indentation and tabulation.

This is indentation: indentation implies the levels of nested structures.

void f(){
    while (1){
        if (a == b){
            printf("something");
        }
    }
}

This is tabulation: tabulation makes multiple lines aligned wrt. their contents.

int apple           = 1;
int foo             = 3;
int another_brick   = 4;
int in_the_wall     = 42;

# End the war once and for all

• Indentation should always be TABs because one is exactly one level of indentation.
• Tabulation should always be SPACEs

A handy script:

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command! RetabIndents retab!|call RetabIndents()
func! RetabIndents()
    let default_tab = &expandtab
    set et
    let saved_view = winsaveview()
    execute '%s@^\(\ \{'.&ts.'\}\)\+@\=repeat("\t", len(submatch(0))/'.&ts.')@e'
    let &expandtab=default_tab
    call winrestview(saved_view)
endfunc

this script replaces tabs inside of lines with spaces and heading spaces with tabs.

This is a modified version of
https://stackoverflow.com/a/5173322/5704060