An acute dilated pupil signals dysfunction of the third cranial nerve or sympathetic/parasympathetic pupillary pathways. Life-threatening etiologies of third nerve cranial impairment include uncal herniation, cerebral aneurysm, or another space-occupying lesion. Consequently, the finding of an acute, unilateral, dilated pupil warrants immediate clinical evaluation and neuroimaging. However, anisocoria without findings of oculomotor deficits is rare in compression-induced third cranial nerve palsies but is rather commonly associated with pharmacological etiologies or Adie’s pupil. Specifically, anticholinergics allow unopposed sympathetic stimulation of the dilator pupillae muscle, resulting in a dilated pupil. Administration of ipratropium through a face mask inhaler is common in intensive care respiratory management. However, masks are susceptible to leakage and displacement. Pupillary dilatation secondary to ipratropium has been described but is not widely known. We presented a case report where an unilateral anisocoria is secondary to contamination of ipratropium to the right eye.