Skirting the boundaries of tax rules is a profitable activity for financial institutions — for a while. But you can easily get caught out when the political and legal environment changes. Deutsche Bank’s mistake was to heed the pleas of bonus-hungry bankers and ignore the warnings of its own tax experts.

The bean counters said the German bank could end up breaking the rules if it became involved in the lucrative “cum-ex” trading merry-go-round. An internal investigation has concluded that was exactly what happened afterwards.

The international tax system has more loopholes than a medieval castle. New ones appear even as old ones close. Examples? For many years, British brokers offered “bed and breakfast” services. Clients used these to sell and repurchase portfolios to take artificial advantage of capital gains tax reliefs. Then there was Barclays’ famous structured products unit, which made huge sums from tax-based deals in the noughties. Finally, modern private equity is adept at shunting risk capital on to the tax-deductible debt side of the P&L.