[pbhjpbhj]

You don't need much in the way of resources to invalidate a patent. Proof of prior use, or published prior disclosure. The measure of proof being balance-of-probabilities.

If you have that you're immune, show up with your print out, you get costs.

If you lose you might have to, for example, hand over profits.

The flipside is that small companies can use patents to prevent unlicensed exploitation of their own inventions.

This is my personal view and does not relate to my employment.

[clusterfish]

That is absolutely not the case, at least in the US. The people who have actually been frivolously sued and managed to invalidate the patents say differently: it takes several years and millions of dollars to invalidate just a few claims in the patent despite ample evidence. Look up the case against Laminar Research for example.

[zoobab]

In the case of the UPC, the court fees to invalidate a patent are already at 20K EUR. Not to mention the additional lawyers fees.