What's in your .vimrc file?
rarrrrrr:   "tab completion of words in insert mode
  function InsertTabWrapper()
        let col = col('.') - 1
        if !col || getline('.')[col - 1] !~ '\k'
            return "\<tab>"
        else
            return "\<c-p>"
        endif
  endfunction

  inoremap <tab> <c-r>=InsertTabWrapper()<cr>