Yes, you have to be careful about offering free advice.
I have known cases where folk have come back for a second (...third) bite of the cherry on the grounds of:- "well, you helped us last time"!
As well as, naturally, "getting the blame" when things didn't turn out as hoped (even though you had zero control over how - or even whether - your advice was implemented).
But worst of all is when someone takes your free advice (sometimes under false pretenses - masquerading
as a charity or what-have-you - or even a "researcher") only to use it in some money-making venture.
As you know, Neil, myself I like to limit my "advice" on here to those I feel obliged to try to help ... generally biomed techs labouring in "less than ideal" conditions, usually overseas. But to be honest, now that almost everyone has access to the internet in one way or another, I'm not sure even those guys really need much "advice" these days.
What they probably actually need (as has always been the case as far back as I can remember) is a helping hand in the physical sense; "boots on the ground", if you like. But there again, we're almost certainly talking about another "charity" (aka
working for nowt) situation there, as well.
Never forget:- "No Good Deed Goes Unpunished"!