As the share price of BHP Billiton Limited (ASX: BHP) has plunged to lows not seen in more than a decade, investors have no doubt also recognised that the miner’s dividend yield has soared through the roof.

It’s what some analysts described as a “once in a lifetime opportunity” back in August last year, referring to the fact that it is so rare that any company would offer such a high yield, let alone a company with the size and market following of BHP Billiton.

The shares were changing hands for around $24 at the time with The Sydney Morning Herald noting a dividend yield of “about 7 per cent”.

As fate would have it, the shares are now offering a staggering 11.6% fully franked dividend yield, equating to a 16.5% annual return when grossed up for the franking credits. The shares are down 0.3% today at just $14.58, slightly above their recent low of $14.45.

The issues that have dragged BHP’s shares lower have been well documented, including a disaster at one of its mines in Brazil (which is tipped to cost billions of dollars to clean up) as well as heavy falls in commodity prices – most notably iron ore and oil. The problem is, as hard as these commodities have fallen, they’re still expected to drop even lower by most analysts, which should further weigh on the miner’s earnings.

What investors need to realise is that an 11.6% fully franked dividend yield is by no means guaranteed. It is, in fact, based on the company’s payments to shareholders in the 2015 financial year (US 124 cents per share), which equated to roughly $1.68 per share when translated to Australian dollars. In other words, it was bolstered by the weak Australian dollar.

Now, the Australian dollar has fallen even further since then, and that’s good for local shareholders. What is bad however, is that the miner may not be able to maintain that US 124 cents per share payment.

Although it has committed to increasing, or at least maintaining its dividends, the headwinds are taking their toll on BHP’s earnings and cash flows. The recent US$7.2 billion impairment on its Onshore US oil assets highlight some of the damage inflicted, while it could also lead to a review of the miner’s credit score – BHP would prefer to cut its dividend before it put its credit score in jeopardy.

Foolish takeaway

Falling share prices can create amazing dividend opportunities, but investors need to be careful. Slater & Gordon Limited (ASX: SGH) is another company trading on an impressive trailing dividend yield, although it’s unlikely to sustain it, together with Monadelphous Group Limited (ASX: MND) and Worleyparsons Limited (ASX: WOR).

I wouldn’t put my money on any of those companies right now, and think investors should look elsewhere for superior opportunities.

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Motley Fool contributor Ryan Newman has no position in any stocks mentioned. Unless otherwise noted, the author does not have a position in any stocks mentioned by the author in the comments below. You can follow Ryan on Twitter @ASXvalueinvest.

The Motley Fool Australia has no position in any of the stocks mentioned. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy. This article contains general investment advice only (under AFSL 400691). Authorised by Bruce Jackson.