First came the hawkers, thrusting flyers through car windows. Next it was musicians, gyrating atop carnival floats in the city’s gridlocked streets. Finally, the battle of the banks took to the sky: a lone helicopter buzzing overhead in the most extravagant share-marketing ruse of all.

For much of last year, it was impossible to cross Lagos without being bombarded by invitations to buy a stake in one of the fastest-growing banking industries in the world. But with stock prices starting to shed some of their J-shaped gains, and many Nigerians getting their first taste of a falling market, investors are taking a cooler-headed look at the sector’s potential pitfalls.