That's unluckily exactly it. Although common sense tells you that nobody can just trademark or patent "blue hedgehog" or "the letter A" or "multiply two numbers", people have very successfully done just that (the colour magenta has been patented in my country...). Common sense does not apply here and it does not protect you.

Thing is, even if a claim is total bollocks, it is still entirely possible that you have to pay a 5 or 6 figure sum for court fees in the worst case (and if you can't afford that, it's just bad luck).

It gets worse if there is the actual or alleged possibility (due to close resemblance) that whatever you do could be confused with an existing competitive product. Because in that case, in addition to the already painful court fees, you are very likely to lose the lawsuit. If that happens, you can as well spend your last few dollars on a gun. It needs not be a pretty one, you'll only use it once.

All in all, you are never safe from lawsuits (except if you base your company in.... one of those countries, you know...) but the point is, you can ask for a lawsuit or you can not ask for one.

That was pretty much what I expected to learn. Ugh. Common sense never seems to apply when it comes to laws. Funny how that works.