Trees rings were reliable for thousands of years, but (like everyone else) they quit doing their job properly in the 1960s. Fortunately, we have conscientious climate scientists like Michael Mann to show the trees what they should have been doing.
Not to be outdone at hiding the decline, Hansen and USHCN showed that they could alter the data in-situ, when they turned a US cooling trend into a warming trend. This appears to be a superior approach to Mann’s grafting on a different data set.
Honorable mention goes to IPCC – which grafted altimetry sea level data on to tide gauge data – to show an increase in sea level rise rates. Apparently tide gauges also went on strike in 1994.