This is one of the handful of places where whitespace matters. In short, it’s the same as the difference between [1 + 2] and [1 +2]:
julia> [1 + 2]
1-element Array{Int64,1}:
3
julia> [1 +2]
1×2 Array{Int64,2}:
1 2
In the latter case, the + becomes a unary operator and you get a horizontal concatenation (that’s why hcat is showing up in the error).