Recently, org-mode made a non-standard tab-width an error state.
Unfortunately, it's way too easy for users to accidentally change
it (e.g. with editorconfig, indiscriminate setter hooks, or other
packages), and since there is zero reason to ever want a non-standard
tab-width in org-mode, I not only exclude it from dtrt-indent as a
global default (i.e. not only as part of :lang org), but I add a late
org-mode-hook hook to reset tab-width, just in case.