I have cinnamon raisin bread dough (hopefully) rising in a cold oven right now and I did something bad. I subbed half-and-half for milk.
Now, I have played fast and loose with the liquids called for in various recipes before. I've subbed soy and/or almond milk for regular milk in scones to no ill effect (and actually I might have made it a bit more vanilla-y, since the almond milk was vanilla flavored). I've also subbed different milks (skim for whole) and have not noticed a detrimental effect.
However, half-and-half is another beast. The fat content is higher, so it could actually affect whatever magical chemistry is going on in my bread. To make matters worse, I only had 1 cup of half-and-half and the recipe called for 1 1/2 cups. So (instead of waiting until I had all the right ingredients, like normal people do) I scaled the entire recipe down by a third, resulting in odd measurements like 5 1/3 cups flour, etc. I've heard that bread recipes don't always scale or halve well, but I crossed my fingers and threw caution (and flour) to the wind. Besides, if I hadn't used that half-and-half by today it probably would have been on its way to funkytown (anyone recognize the quote?). So I HAD to use it.
I've got probably another hour of rise to go so we'll see what happens. For some reason I got a bug in my head about raisin bread and all I wanted was raisin bread and holy cow Pepperidge Farm raising bread is like $3.99 at the grocery store and I can do better than that and look the special baking raisins are on sale for a dollar and...well...turning and turning in the widening gyre, I suppose.
Wish me luck! I will report back (if I remember) on the results of this daring substitution.