Ratio chart between the S&P 500 and US Treasury Bonds. The ratio is right now at the same height as before the crash in 2008 and it could eventually reach the levels of the pre-2000 crash if it continues.
The only times the monthly RSI(14) of this chart went over 70, a crash eventually followed. However notice the period from 1994 to 2000 where RSI hovered around 70 for a long time before there was a crash.
Take this chart with a grain of salt. The current fundamentals are much different then in the previous crises and a real crash is still unlikely. However, embracing for a good a legitimate correction in the stock market is reasonable.