Edward Whittingslow

Thank you for reporting this, at first I wasn't sure this was a bug. I have a cow pen using only a wooden fence perimeter, and I noticed that after breeding the cows - which made the cow pen a bit more crowded and congested - when I left the area a good ways and later returned home, there would sometimes be 1-3 cows outside the pen walking or standing about. However, I had so many cows in my large pen, I was not sure if they were my cows, or randomly-spawned cows. (Cows did spawn in the area prior to me building a house and pen on the land). Unfortunately, I am ignorant to how the mob spawning system works in Minecraft, and I thought maybe "wild" cows were continuing to spawn, or that a large group of cows could possibly trigger the spawning of random "wild" cows. Oh, and in previous builds of Minecraft I've seen animal mobs (pigs, cows) walk/push half-way through fencing before, but never actually get through the fence or take damage from it. From this report I guess I can assume the "stray" cows are mine episodically glitching outside the fence when I explore outside the vicinity of my pen and then return home.

It would be nice to see this bug removed, I can't imagine it is too difficult. The only work-around is to make at least 2 buckets and stack them, and when you right-click on a cow one of the buckets will fill with milk, leave the stack, and show up in a new inventory slot - assuming one is available. I'm not sure what would happen if your inventory was full, I did not test that scenario. But I agree, losing 3 iron when you forget to stack buckets to get milk is a real annoyance.

It is not the launcher; I used the "dev" launcher set to 1.5.2, as well as the "default" launcher for build 1.5.2 and was able to fill buckets with milk without an issue. However, any snapshot build after 1.5.2 (using the dev launcher, naturally) results in a loss of a bucket.
I have only tried with a single bucket, not stacked buckets.