A friend of mine just made a small batch of salsa and put in 5 habanero peppers. She wanted it to be smoking hot but to her dismay it came out mild. It did get a tiny bit hotter after a couple weeks but still is mild. Any idea why it didn't turn out hotter?
- Ruth

I can think of a number of reasons why your friend's salsa ended up not being as hot as expected:

1. According to Harold McGee in On Food and Cooking: The Science and Lore of the Kitchen, the majority of the heat in peppers resides in the central pith of the fruit, and some in the seeds.  There is also some of the active ingredient, capsaicin, throughout the flesh.  In a lab test on jalapenos, Cook's Illustrated found that nearly 87% of the capsaicin was in the pith, around 12% was in the seeds and just under 1% was in the flesh (September - October, 2013).  If your friend was very careful removing all of the seeds and pith, then it is possible that she discarded most of the heat along with them.  The amount of skill this would take, though, is formidable!
2. The amount of heat in peppers depends on a lot of factors, including soil, weather, and age.  Once peppers mature, they start to lose heat so older peppers will not be as strong as ones that are just ripe.  Hotter, drier weather conditions make for hotter peppers.  It is possible that one or more of these factors may have affected the peppers used, although I would expect them to still be hot.
3. There are a number of peppers marketed as sweet habanero type peppers.  These include, among others, Zavory, Trinidad Perfume, Tobago Seasoning and Aji Dolce.  All of these measure in the low hundreds on the Scoville scale, putting them in about the same range as a regular bell pepper (See the post Measuring the Heat of Peppers for a description of Scoville units).  It is possible that the source mislabeled Sweet Habaneros as simply Habaneros.
4. There are also marketed nowadays Sweet Mini Peppers.  Your friend may have used these thinking they were habaneros.
5. I add this only last possibility because it is a mistake I have made during a "seniors moment" -- could your friend have picked up jalapenos thinking they were getting habaneros?

The most likely possibility is that your friend bought sweet habaneros that were sold as regular ones.

One suggestion I would make is something I almost always do when working with hot peppers.  After I have split and cleaned the peppers, and prepared then for the dish I am making, I cut a tiny piece off, about the size of a pin head and taste it raw.  If it is really hot, I know to be moderate and add a little at a time until I get the heat I want.  If it is bland, then I know I can use lots.  In this particular case, your friend might have known from the beginning that the peppers wouldn't be hot enough.