Edit: duplicate of After 4.2SP1 upgrade img tags are broken
This is just for your information.
I had images in content blocks. I didn't specify an alt tag for them, so SF generated an "alt" attribute with no value.
After updating from 4.2 to 4.2 SP1, the HTML of all of my content blocks has been updated.
This ruined my markup, causing all kinds of problems.
Too bad the missing double quotes didn't make it.