Bubble about to burst?

This is not a trade idea, just an interesting observation. This spread chart is essentially the difference between the S&P 500 and the Russell 2000 (SPY-IWM). The last time it was at this level, it was before the crash in 08 (additionally, it was very high before the dot-com crash). Not saying this is the market's destiny, but it might confirm the thesis of a correction.

I'm still bullish about this market over the short-term however and I'm especially bullish on USD.

But let's be aware of this pattern.