I have heard some people suggest that this is a sign they are ready for a larger prey item. I guess it is less common for them to make that mistake when the head of their food is noticably larger than their rump.Then again, it seems like some just aren't that picky lol. I definitely wouldn't worry about it as long as they are eating though. You may try a larger rat once a week instead of two smaller ones depending on the age and size of the snake. I have started feeding my guy by dangling the rat by the tail slightly above his nose so that he will grab it in the right position for swallowing. I was having some trouble with him striking in the middle of its body and then getting confused and spending the next half an hour trying to reposition in a way that he could swallow it. This probably caused me more stress than it did him, but we have had fewer problems since I have found a way to get him to grab it by the head. These days he almost always eats normally and gets it down without incident.