Has anyone ever seen this behavior before? I seem to have random entries missing from my site. This has been going on for a few months now and I've chalked it up to some weird anomaly when it first started happening. Now, I'm noticing that entries are missing because I'm getting crawl errors inside of Analytics and Adsense admin. I probably haven't deleted an entry in five years so something weird is going on.
This is NOT good. A couple of possibilities occur to me.
Your host migrated your site to a new database server and failed to import the data correctly.
Someone has exploited a vulnerability in EE1.
If you look at the EE1 Change Log there are security fixes right up until the final version (1.7.3). If you are running a version older than that there are certainly vulnerabilities. If you are 1.7.3 there are very likely vulnerabilities as the code is not longer being maintained.
I'd start by going through my server log files to look for unusual control panel access, but some vulnerabilities might not even require this.
If you have database backups I'd look at those as well to see if you can determine when the missing data disappeared.
Updating to the latest version of EE1 might help but for the longer term you must get off of EE1 for the sake of security.