Might be important

3913 prior support that we failed to hold below. My line in the sand for continued selling. No coincidence that we have significant gamma sitting in that area as well

Comment:

So many people saw this level via different methods. Structural (like shown above) but also significant options positioning around there and $390 on $SPY lead to it being front run a bit this afternoon.

Comment:

Finally tested, we'll see if it holds tomorrow