Long story short: Does the user input contain 'strange' Characters, WordPress behaves relly strange. To examples:
1) Inserting content with 'strange' Characters, will only insert content up to the last non-'strange' character
2) I have a plugin, that stores arrays in custom fields. Do the arrays contain 'strange' characters the hole field gets corrupted (I think something like the serialised version of the array breaks right before the first strange character would appear). This is relly bad, because the hole array disappears, not only some content.
3) The content of a post just strips the srange characters when saving.
To reproduce, try it with the following content: http://www.continent7.com/uploads/test.txt