I've put a tiny piece of blue tape on Blu-ray disc with data, and, surprisingly, it was still fully readable (reading speed dropped at this place, that's all).

Next, I've put a slightly bigger piece of tape on it, and it made ~15% of disc unreadable. Reading "damaged" areas is quite slow, so I interrupted ddrescue/dvdisaster when data reading speed dropped too low, and it was around 15%.

Both results are strange to me. First is almost too good to believe, second is too bad. Maybe data behind little piece of tape have been recovered by internal Error Correction Code? And big piece, sure it didn't cover 15% of readable surface. Maybe full "recover" by ddrescue would have copied a lot more data, but I doubt it.

Why do you put tape on your disc?

I was trying to proof DVDisaster application works and will be able to recover missing data. I've failed to do so.