I just have learned something about the inline assembler's labels: I am using routinely L1%=, L2%= etc. and have no problems so far (sort of boring, maybe, but those assembler snippets tend to be short and relatively similar to each other, so I saw no need to be more inventive in the names).
Now, in two different routines I used L11%= in one, and L1%= in the other.
It happened, that %= got replaced by a unique number for each asm statement, as the documentation says - particularly, by 39 in the first, and by 139 in the other. It means, that I ended up with two L1139 labels... which of course results in assembler error.
Somebody more competent than me might perhaps add this small snippet of information to the Cookbook/avr-libc documentation, for the benefit of the future generations...