When parents have multiple children, it's common for them to be concerned that they'll end up showing one preferential treatment over the others.
And since this is something parents can easily do without realizing it, we can't exactly say this concern is based on nothing.
It's also true that those who received the short end of parental favoritism tend to develop a keen ability to recognize similar dynamics in other situations. So when the matter comes up, discussions about it can get pretty heated.
And that's likely why one mom's simple question on a parenting forum ended up attracting such a passionate debate.