I just tested this on all my event logs I still have (dating back till jan 2003). There is nowhere anything like you described.... Such a bug or anything remotely similar is also never reported by anyone before.
So I very highly doubt that this is due to Messenger Plus!
What you described is most likely a mock up on the part of your computer and nothing more (trust me, I've encountered such things like you described on many occasions since I first started to use a computer 20 years ago). It is well known that when the hard disk gets corrupted slightly, the FAT gets corrupted or file chain links got messed up, that all sorts of files can be get messed up and include parts of other much used files (eg: like cache files from your browser).
Moreover, Messenger Plus! "logs" absolutely nothing what you say in your chat or whatever! There is no security issue at all, as Messenger Plus! will not watch what you type and send that to its servers or whatever. Everything is saved locally on your computer. Messenger Plus! absolutely does not watch what you do on your computer!
(For the record, the only thing it does log and send back are numerical and totally anonymous statistics of what features you use of Messenger Plus!, nothing more. And that can even be turned off, btw)
How strange or concearning it might seem to you, this all looks like a messed up hard disk (corrupted FAT, corrupted relocation table, broken chain links, crossed links, bad or lost clusters or sectors, etc etc), nothing more*. So, instead I suggest you do a proper and thruout test for possible errors in the file system and for bad sectors and clusters and fix your hard disk, if this is still the hard disk you used at the time those files where saved.
* "nothing more" is of course an understatement. If you experience such errors it means there is something seriously wrong with your hard disk and/or file system and needs to be fixed ASAP.
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On another note, be aware that there are virusses, trojans and other malicious stuff which also can corrupt files like you described.
Also, there are fake Messenger Plus! versions on the net to abuse people and to make Messenger Plus! look bad. Never download and install Messenger Plus! other than from the official site!!!!!!
Also, check your Plus! plugins. In the new versions of Plus!, plugins are able to write something to the event log also. If they mock it up, it is also not the fault of Messenger Plus!, but from the bad written plugins (which dump random memory contents instead of parsing the proper string to write to the log). But again, this is not spying or whatever and is just a case of what is on that particular memory address at that time. Such bad written plugins do not know what is located at those memory addresses, neither do they "log" anything.
If you would look at the contents of your computer memory you would see that there is all kind of stuff "which you did". And that is never reset to zero, instead the locations are marked as reusable and are simply overwritten with new stuff of "what you just did". This is how memory works, in a nutshell and in general.
The above paragraph goes also for any other addon you might have for MSN Messenger.
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To conclude, I am 99,99999999% certain that the things you describe do not come from Messenger Plus!. It simply does not make any sense at all and there are far more (and more logical) things which can cause this...
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PS: please don't double post though, if you want to add anything to your post, edit the latest post instead.
Also note that this isn't a chat room where you can expect an instant answer (though, the response time given on this forum is on average extremely quick)