SGI MIPS, Speculative Execution issue

hi
"Speculative Execution"  is a feature of the R10000 Processor.
I read that is problematic on machines that are Non-Cache Coherent,
such as the IP28 Indigo2 and on R10000/R12000-based IP32 O2 systems.

The first thing I don't understand is what does it mean that they are
"Non-Cache Coherent" systems.

I mean, I know what coherence of the cache means in a multiprocessor
environment, but I'm a bit confused by what I read for these mono
processor systems.

A few more details on the nature of Speculative Execution, and the
issues it poses to the Indigo2 can be found at the following URLs:

1) MIPS R10000 Microprocessor User's Manual (pages 51-55 )
https://web.archive.org/web/20051028113506/http://techpubs.sgi.com/library/manuals/2000/007-2490-001/pdf/007-2490-001.pdf

2) Post to NetBSD sgimips Mailing List on 29 Jun 2000
http://mail-index.netbsd.org/port-sgimips/2000/06/29/0006.html

however there are no examples.

Can someone explain this matter to me?

Thanks