From Poland: obligatory non-renunciable 24-month warranty of fitness for the intended purpose for consumers. Instructions required in Polish. Detailed information about qualities of goods sold is compulsory (e.g. specification of components for a computer, just model number is not enough).

From Germany: specific requirements for "impressum" on each website. Definition of all quantities in SI units.

Things get more funny if you touch any food: labelling regulations have a common EU base, but definition of what detail level is required for each element is per country, and they do differ significantly.

For everywhere: obligation to collect and report VAT for each destination country.