So I had gotten out of the habit of checking the account statements for my Fidelity Investment account because I had set up 4x a month auto-transfers from my bank account that automatically purchased shares for my S&P 500 index fund. The amounts were regular, the timing was regular, my portfolio kept regularly increasing, and everything seemed good.

Last July, I had a $9500 CD mature and decided to deposit the funds into that index fund investment account. My account balance rose as I expected. I attempted to do something similar last Friday with the recent stock market correction, depositing another $2000 into that index fund investment account and betting that the S&P 500 would go back up soon or eventually.

Only when I decided to spend a bit more time looking through my account today on Fidelity's website did I remember that my investment account has both the index fund as well as a 'Core' money market account that functions as a temporary wallet to hold money. And staring me in the face, was my 'Core' account balance was of just over $11,500.

I thus missed two great opportunities to purchase Index Fund shares at a much better price than they are at today, March 2nd. If I had reviewed my bank statements, I would have wondered why a large chunk was not sitting in my actual index fund and would have remembered to actually purchase the fund shares. I put in an order tonight to use the full amount to purchase the index fund shares first thing tomorrow morning, so we'll see what the market does...

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TL;DR: Forgot that transferring money to an investment account doesn't automatically buy shares of a fund in that account. Failed to buy shares of an index fund at good prices, so the money sat in a 'Core' account for 9 months in essentially a savings account.