Things I'm unhappy about are for instance
f(x) \in L(\R) where f \in L(\R) is meant
F(x) = \int f(x) \dif x where x shouldn't be visible outside the integral
O(n) which should be O(\n -> n) (a remark by Simon Thompson in The Craft of Functional Programming) f(.) which means \x -> f x or just f
All of these are the same notation abuse, "sometimes f x is meant to be interpreted as \x->f x" In some cases it would be really tedious to add the extra lambdas, so the expression used in its definition is used to denote the function itself.
a < b < c which is a short-cut of a < b \land b < c
Both, ambiguity and complex notation, can lead to (human) parsing problems, which is what we are trying to minimise here. J.A.