Safari occasionally zooms right in on a web page. It's quick to return to the actual size, but you can also disable this behavior if desired.
Myself and friends/family who I support were being bothered by Safari occasionally zooming right in on a web page. Pressing Command+0 (zero) undid it, but it was still disconcerting.
I found this is caused by Safari on OS X trying to emulate Safari on iOS: a double-tap on the mouse zooms the page to the current column.
Take a trip to System Preferences to disable this behavior: it will be either Mouse » Point & Click, and turn off Smart zoom, or Trackpad » Scroll & Zoom, and uncheck Smart zoom.
[crarko adds: This is one of those things I never see since I disable Smart zooming as a matter of course on all of my systems. I have met a number of people who do encounter this phenomenon, and asked what to do about it. So it's not an earth-shattering hint, but perhaps less obvious than it seems. I do think pinch zooming on a trackpad is useful and a logically consistent thing to do. Do folks here make a lot of use of double-tap Smart zooming on a Mac? Knowing the circumstances where it proves useful would be a nice addition to the tip given here.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20130108131950239