The directions for a laboratory activity call for #50# milliliters (#"mL"#) of solution A. A student accidentally takes #"55 mL"# from the stock bottle. What should the student do with the extra #"5 mL"# of solution A?
What the teacher doesn't see, the teacher doesn't know.............
So if it's an aqueous solution, just tip it down the sink. Of course, it's your own risk whether you get observed or get grassed up.
My own very conservative notions of lab management would insist that you don't add chemicals BACK to a stock bottle when they have been dispensed on the basis of the risk of contamination of the stock solution.