Once I had downloaded a HN thread for later reading, on my iPhone with Safari's built-in Download Linked File feature. When I opened it and was reading it (HN renders fine with just the HTML), I noticed a single line looking a bit off. Took my glasses off to take a closer look. I don't remember the specifics, but I think the spacing with the line above was slightly less than usual. Took screenshots and it was clearly visible zoomed in. Happened twice that day and never before, never after. I believe I witnessed a cosmic ray induced bit flip.