Can ordinary DVDs harm Blu-ray players? NO

[who007]

Hey,
I bought a bluray dvd player (Samsung BD-P1500 1080p Blu-ray Player)
and watched two ordinary DVDs on it (I haven't bought a bluray dvd as yet) and the quality was really good with no issues. The next day I tried to watch another and when I placed the DVD in the machine I got the message LOADING on the display - which didn't change so eventually I thouhgt it might have been the dvd. I tried the original dvd I had watched and it too didn't get past the LOADING message.
So I got the player replaced today but when I was opening it there was a sticker I hadn't previously noticed saying not to play ordinary DVDs (or some such words - am not at home right now).
Is there some difference in the laser layout or something that may have caused damage when I played ordinary dvds in it? I got it as part of a package and wouldn't have bought a blu ray machine otherwise, so don't have any plans to go buying bluray discs as yet.
Long and short of the question: Was this a coincidence or can I or can I not play ordinary DVDs on a Bluray player?