Is there any chance I can damage a DVD drive by inserting a Blu-Ray disc into it? Can I damage a CD drive by inserting a DVD or Blu-Ray disc into it?
Also, can a disc drive damage a disc it can't handle?
The simple answer is that yes - it can cause damage, but this is very unlikely.
The difference in power between the lasers in DVD/BluRay/CD readers and writers is negligable, but in some lower quality hardware, the power of the laser diodes can cause damage to the lens and pickup assembly when badly reflected or refracted.
In all honestly, I wouldn't worry about it.