Backend is hdb. The add was done under 2.3.37, the server has since been upgraded to 2.3.40 (last Thursday).
db_dump -p does show a high character...
\80\10cn=172.23.58.210\00cn=1\b72.23.58.210\00\00\00\00\00\00\01\e4e
which I suppose you're theorizing is off-by-one on the disk. BTW, slapcat doesn't agree with ldapsearch here, which might be construed as a bug? (Although it's tough to call something a bug in an impossible situation, I admit.)
I dunno. I suppose if the hardware is bad then I might be seeing all sorts of crazy stuff. But we're talking modern sparc/solaris hardware: all my other boxes are pretty good about screaming during their death. And they also don't have the amazingly-coincidental upgrade to 2.3.40 from last Thursday, either. (Which I started seeing err=80 returns within 24 hours of, after not seeing them...well, almost ever in production.) It's all just a bit too perfect a storm for my taste...but I'd obviously defer to you when it comes to bdb internals.
With that in mind, I might as well see through your theory. I'll play with a second box tomorrow and see if I can reproduce fresh.