All:
TL/DR: megadose biotin of the sort that has become common in hari-skin-nails products and many "premium" multivitamins can interfere with a broad range of blood test results, including thyroid tests, cardiac markers, cortisol, and testosterone. Full post here. (Please do not just copy-paste this).
Obviously, we should try to remember this and skip any biotin supplementation the day before a test (and not take them the morning either — in general, one probably shouldn't be taking any supplements anyway, just to be sure. Antioxidants, for instance, can interfere with the glucose oxidase test for blood glucose, although it's my understanding that this test is not widely used any more). This will, of course, be PITA if one's multi already contains a lot, and one should set up some kind of mnemonic or one will probably forget.