Over the period 1964-1970, the suicide rate in England fell by about one-third.  During this period, a volunteer welfar organization called "The Samaritans" was expanding rapidly.  One investigator thought that the Samaritans were responsible for the declien in suicides.  He did an observational study to prove it.  This study was based on 15 pairs of towns.  To control for confounding, the towns in a pair were matched on the variables regarded as important.  One town in each pair had a branch of the Samaritans; the other did not.  On the whole, the towns with the Samaritans had lower suicide rates.  So the Samaritans prevented suicides.  Or did they?

Another researcher thought they didn't do a good job of controlling for variables.  A second, more careful study found no effect.  Further, during this time period in England, gas ovens were phased out in favor of electricity.  About 1/3 of all suicides before 1964 involved gas ovens, 0% of them after 1970.   So quite possibly the declining suicide rate was attributed to that.  In support of this theory, the rate of suicide due to other methods remained constant in 1964-1970 and even increased after 1970, despite the fact that the Samaritans continued to expand.