Steps to reproduce:

1. Edit a page in the visual editor.
2. Insert an image, such as [[File:Sanxbox.JPG]], with the default settings.
3. Change the image's caption to left|This is my caption.
4. Save the page.

Expected output:

• The image is exactly as it appeared whilst editing: a thumb, on the right of the page, with the caption left|This is my caption.

Actual output:

• The page is different from it appeared whilst editing: a thumb, on the left of the page, with the caption This is my caption.

This happens because changing the caption to left|This is my caption. changes the wikitext of the image to [[File:Sanxbox.JPG|thumb|left|This is my caption.]] which then has the obvious effect of moving the image to the left.