By chance the other day I came across Spurious correlation at the FRED Blog. They write:
Relationships between macroeconomic time series are not usually straightforward enough to establish with a simple graph. The problem is that almost all time series tend to grow in the long term as an economy grows... Because time series can exhibit a common trend, it becomes difficult to interpret whether there is a relationship between them beyond that common trend. We call this spurious correlation.
By chance just now I came across an old one of mine containing an excerpt from Robert Lucas's Nobel Prize lecture. This excerpt:
Figure 1, taken from McCandless and Weber (1995), plots 30 year (1960-1990) average annual inflation rates against average annual growth rates of M2 over the same 30 year period, for a total of 110 countries. One can see that the points lie roughly on the 45-degree line, as predicted by the quantity theory. The simple correlation between inflation and money growth is .95.
For Robert Lucas, 110 spurious correlations on one graph is good evidence.