Now we are getting into a whole different field. However it may be worth seeking some authoritative guide here - you have not defined the words "knew" "risk" or "fail."
Courts nearly always give judgements on "risks" where there is a measurable chance that harm may be caused - how much harm is likely to be caused if a floor tile comes unstuck? Should it be judged on the same basis as deliberately using electric cable insufficient to cope with demand leaving a measurable risk of fire and death by electrocution?
When you look at whether a tradesman "knew" something would happen, you apply a test that something reasonable has happened in the experience of that tradesperson to give them that knowledge - good practice may suggest that tiling on tiles is not good practice - but does that mean the work will fail? What tests had the tradesperson conducted to confirm this belief? If you go ahead and pressurise a customer into funding a course of work that may not either be necessary or worse, force them to spend money on unnecessary measures?
The there is the question of failure. Over what period of time is a failure unreasonable? 1 week? 1 year? 10 years? 25 years? Is it necessary for us to produce tilling that last over a thousand years every time we do a job?
What I am saying here is that we are not lawyers, we are trades(wo)men. Every customer and every prospective job must be treated on its own independant merits. It may be entirely inappropriate to tile on a tiled surface in many cases, but is it right to say "never do it" - even the British (and European) Standards are "advisory" and if you have good reason not to comply with them - then do so - but document why, and be prepared to justify it.