Investors like me who merely seek to replicate the returns of a broad index and not to time markets but merely passively track the index often use ETFs to do so. It's probably no surprise that ETFs vary considerably in how well they do the job of tracking the target index. The measure of the deviation from the index is tracking error.
Forbes' ETFs Behaving Badly article and accompanying 20 Best and 20 Worst slide shows describes results of a survey of 505 US-traded ETFs done by Morgan Stanley for 2008. In many of the worst cases the tracking error is several percentage points. The best have really tiny tracking errors.
Many of the worst trackers turned out to have out-performed or done better than the index in 2008. The article explains how some of those came about which gives me the sense that it's likely to keep happening. It's perhaps a nice accident that some results were better than the index in 2008 but in future years an uncontrolled or uncontrollable tracking error could well mean serious under-performance. Just give me the index please!
Most of both the best and worst lists are quite specialized ETFs. It's reassuring to see that among the best are Vanguard's Total US bond market ETF (BND) and iShares US TIPS Inflation-Indexed Bond Fund (TIP). A surprise is that some of the worst are several Vanguard offerings like their Energy Fund (VDE) and a Telecomms Fund (VOX) and an ETF heavyweight, iShares MSCI Emerging Markets Fund (EEM). There are also several bad country trackers, notably iShares' ETFs for Mexico (EWW) and Austria (EWO) and the SPDR S&P China fund (GXC).