Odd things happen when saving document

This doesn’t happen often, but it does occasionally happen, and I have no idea why it happens. I tend to notice this usually occurs near the most recent paragraphs typed in just before I save.

So I am writing stories and will type in a line:

Tom hid his head in Steve’s chest.

Save the document and close down for the night. Next morning I open the document and the line will now read:

Tom hid his have in Steve’s chest.

The most common time this will happen is the last couple of paragraphs after a save and after closing down for the night. However, it has also happened after saving a document and then a week later will go back and re-read the entire story I have written so far and will notice that here and there, words have been changed. This confuses me as to how and why this is happening.