Region Error doesn't refer to PAL vs. NTSC. DVDs can be marked by the manufacturer with a "Region Number". Region 1 is the United States, Canada, and some smaller nations. Europe is Region 2. A DVD player for Region 1 will not play DVDs from any other region.
When you converted your NTSC video to PAL, you also put them on a new DVD that didn't have a Region 2 code, so your Region 1 DVD player was able to play them.