I will usually also include //HACK comments with a little note as to what is broken or what my assumptions were at the time of writing, for things that I believ

I will usually also include //HACK comments with a little note as to what is broken or what my assumptions were at the time of writing, for things that I believe are likely to cause breakage later on. TODOs are for optional/longer-term things as you described, like inefficient implementations because the current assumption is that it will ever only operate on 20 elements. HACKs are more along the lines of "hey if you're debugging this bit of code, your issue is probably here"