The May 17 Nature suggests that papers in less prestigious journals are less likely to be checked by readers for errors:

Murat Cokol and his colleagues at … Columbia ..identified 596 retracted articles – flaggged as such by PubMed – and found … Journals with high impact factors retract more papers, and low-impact journals are more likely not to retract the, the study finds.  It also suggests that high and low-impact journals differ little in detecting flawed articles before they are published.  … Cokol argues that the larger number of retractions in high impact journals reflects the fact that they receive more scrutiny. 

Not surprising, but pregnant with implications.