I've forgotten the hammer as well.
Thankfully I had my Gerber LMF ii in my fire kit that has a flat point in the pommel to hammer stakes in.
I've also forgotten a pillow. I know it doesn't seem like much but I can tell you trying to use a few t-shirts in a bag is not as comfortable as you might want.
I used to carry a Goal Zero Guide 10 every trip and never used it. Of course when we planning a trip where I could have used it I couldn't find it to pack it.
On one of my trips in Maryland my Niece swatted a hornet out of the air and got stung on her hand. And of course I had my giant First Aid/Trauma kit in the Xterra with some Sting Relief wipes.