The image above is a close-up photo of a DVD-R disc, after recording. The disk is nearly full, there is only 1 mm between the edge of the recorded area (light pink color) and the edge of the disk. I purposely shook some dust onto the disk before recording to show the effects. As you can see, the recorded area fades out (appears darker) around the large dust particle on the left, and there is also a dark trail to the right. There are other blotches caused by dust particles which I blew off with compressed air after recording, but before taking the photo.
Below is another area of the same DVD. After recording, I cleaned off the dust particle which caused this shadow, but the defect in the recorded surface is permanent.
In a tribute to the error-correction capabilities of the DVD format, this particular DVD-R played without any noticible errors. However, another DVD I recorded (fresh from the package, without intentionally adding dust) did show glitchy playback and player lock-up at one particular spot near the end. On visual examination I found a number of dust shadow-spots such as these shown above, near the outer edge of the track. Another 23 DVD-Rs from the same batch (Maxell) played with no errors, so the problem seems to be the dust. I am now carefully examining each DVD-R before burning, and dusting with compressed air where appropriate. I have had no problems since starting to clean the discs.