I was interested in how heavily modulated vinyl could be, so I popped
an old record (Long Hot Summer Night, Jimi Hendrix, Track Records
1968) under the microscope for a look. And what did I find? Two
adjacent grooves clearly broken into each other:

http://81.174.169.10/odds/grooves.jpg

Was this a really common back then, or is this kind of thing a rarity?

I had a friend with a Bee Gees record. On the first song, there was a
very audible pre-echo of the opening moments (it was something very
loud, "Tragedy" perhaps).

I never really investigated it. I presume it must have been print-though
from a badly stored tape, but I guess it could have been a very
badly-cut record.

Daniele