- Not value-raising/betting often enough on the river (definitely #1).
- Folding too many marginal hands getting good pot
odds when multitabling (may not actually be a leak - I generally do this because I don't want to have to make too many potentially 'tricky' decisions at the same time).
- Not shoving enough with the big stack on SNG bubbles (though again, many donks will call you so lightly at the lower levels that this may not actually be a leak either).
But I, much like anyone else, probably don't know what my biggest leak is, because if I truly were aware of it I would have taken steps to resolve it, and hence it wouldn't be my biggest leak anymore.