I wanted Iceape, and I couldn't find it in the Lenny repositories, so I added Sid to the repositories in Synaptic so I could look there. After I downloaded Iceape, I noticed that the Synaptic "upgrades available" button on the panel said "493" (!!!). After some surprise, I realized it must be because Synaptic was now hooked up to the Sid repositories.
So I clicked ""Mark All Upgrades." Was that an unsafe idea, since Sid is the unstable release? I wish I hadn't done it right away. I guess if my system starts crashing unexpectedly, I'll know why.

Mixing Stable/Lenny with other branches of Debian is a bad move.
Consequently it is no longer 'stable'.

Just out of interest did you install all 493 updates from Unstable?

When you want something from unstable you add a sid repo in your sources.list, apt-get update and then you either

a) set apt's priorities so as to get only the package you want from sid and dependencies and you keep the repo.

b) install what you want using the "-t unstable" parameter and when you are done you delete the sid repo and re-run apt-get update.

D'oh. Yes, I did. I guess I can only hope they work.

Sid works OK, I use unstable since several years.
From time to time there is a bug, wich you can resolve by downgrading the package, or wait till it's fixed.
Latest problem was apt => segmentation fault
A downgrade of apt solved the problem, when the bug is fixed, I upgrade apt again.
However I would suggest to stay at sid now.

I have been running Sid with all upgrades for a few years now. Never had a problem I could not resolve by either manually using dpkg and reinstalling a previous release file to make things work until the occasional broken file is fixed.

Congratulations you are now running sid.

If you don't already have apt-listbugs and apt-listchanges installed I would suggest you install them.

I'm already very sorry I tried to upgrade using the sid repositories. I do not know why, but the upgrades removed many important programs, including Firefox and Synaptic. I can't even apt-get install synaptic: it produced some error message about a path and something important not being where it should be.
My system has been torn apart badly enough that I am considering reinstalling. It would probably be easier than cleaning up the mess I made.

That will be your best option.

Done. Lesson I learned: don't get upgrades from the sid repositories if you don't need something in particular. I did need Iceape, but had already gotten it from the sid repositories before this happened. After reinstalling MEPIS, I added the sid repositories in Synaptic again, downloaded Iceape, and then deactivated the sid repositories. What a mess.

Sometimes I wonder why people go with SID. I have used SID and testing for a long time and IMHO testing is a better compromise time/bugs/mailing lists.

Even if you want the greatest and latest, be sure that you have as suggested apt-listbugs, also be ready to fix stuff time to time vs wait for a fix.

Well, that was the first time I thought of looking in the Sid repositories for what I can't find in the Lenny repositories. I didn't know until a subsequent thread about the Lenny backports repositories.

@mrcheeks I just use apt-listbugs and if it has a "grave" bug I wait a day or two before I run apt-get dist-upgrade. Other than that I really haven't had any major issues in the 5 yrs I have ran Debian sid.

@craigevil, ok but lots of people don't have your skills :-).

Many new Linux users, unfamiliar with command line and Linux in general, go straight with SID to have the latest software, that was my point. Many of them won't use apt-listbugs, read mailing-lists posts, etc.