HMRC typically identifies ISA oversubscriptions during its annual data reconciliation process after the tax year ends on April 5th, with providers submitting reports by early June [1]. Most discrepancies are flagged within one to two years due to automated reporting systems. However, HMRC has no fixed statutory expiration for pursuing oversubscriptions. Under standard UK tax law, HMRC can investigate good-faith errors within four years, careless errors within six years, and suspected deliberate fraud or evasion within twenty years [2]. Once an oversubscription is discovered—whether immediately or years later—HMRC can instruct the provider to correct it and calculate any tax owed on excess growth. Individuals should retain records of all ISA contributions indefinitely, as HMRC retains the legal authority to revisit past tax years if evidence of oversubscription surfaces.