I frequently hear "50% plus one", as a way of characterizing a simple majority, often from people who ought to know better.
For the benefit of those who fail to see why that expression, although often heard, is nevertheless erroneous, let us consider a simple example. Say we have a situation where there are five votes. Surely we all agree that 3 out of 5 constitutes a simple majority. However that "50% plus one" rule states that three does not suffice! It says we must have at least 3.5 votes for a majority, and so when fractional votes are impossible then 4 out of 5 are needed to achieve its kind of majority.
Since I have yet to encounter anyone who really wants that "50% plus one" definition of majority, I've come to suspect that those talking about "50% plus one" are simply parroting something they've heard and like the sound of, without ever thinking about what it says. The next time you hear that ill-advised idiom from someone you otherwise respect, perhaps you'll point out that he/she means not what she says.
What she means is "50% plus one-half" (or "half plus one-half"), the correct, albeit rarely heard, way of describing what we mean by "majority".