— श्रीमद्वाल्मीकिरामायणे बालकाण्डे षट्चत्वारिंशस्सर्गः (22)
This shloka is told by the great sage VIshwaamitra to Shri Raama and means the following. Lord Indra humbly performed namaskar to his step-mother, Diti, and spoke these apt words “O Devi! You became अशुचि (impure) because today you slept with your head on that side where you usually put your feet while sleeping”.
Since an अशुचि person is in-eligible to perform any karma, we should be cautious not to perform such mis-deeds. More importantly, even if we get अशुचि, we should immediately take bath and/or do आचमनम् to again become शुचि (pure).
Also Lord Indra teaches us another important lesson here: even if elders like mother, father or teacher commit a mistake, one should not point out their mistakes with arrogance or with dis-respect. Like Indra, one should only respectfully remind them of their good while doing namaskaar to them.