Investors are a fickle bunch and they certainly don’t like surprises. However, Asos’ share price fall this morning looks a bit overdramatic at first glance.

The online fashion retailer plunged as much as 22% after it announced sales growth had slowed and it was increasing investment. Shares were still down almost 14% in mid-morning trading.

Asos said sales grew 26% year-on-year to £137m in the two months to February, not exactly tepid growth but well below the jump of 38% in the previous four months (it called both periods ‘quarters’ – questionable maths right there).

The company blamed ‘adverse currency movements’ in Australia and Russia for the slowdown and said it was going to spend £68m on its warehouses and IT this year, above its previous estimate of £55m. Chief exec Nick Robinson said that will increase sales capacity to around £2.5bn, more than £1bn higher than previously expected.

What with online retail being the next big thing for, like, ever, Asos’ shares have risen stratospherically by almost 2,000% in the last five years and more than 26,000% since listing in 2001. That has pushed the company’s price-earnings (PE) ratio to more than 100, usually a sign that investors expect growth and/or the business is risky.

As Panmure Gordon analysts pointed out today, such an over-enthusiastic valuation ‘leaves little room for disappointment’. Sports Direct, for example, has not been doing too shabbily recently and trades at a PE ratio of around 26.

Asos’ newly publicly-listed online clothing cousin Boohoo.com might want to think about tempering investors’ expectations too: its shares soared as much as 70% on its market debut, after listing at 50p. This morning they fell more than 11% to around 59p.