TL;DR version--do you have apps in your Google Play "Other apps in my library" that you know you've never installed?
In my Google Play account, under the section labeled "Other apps in my library", I'm shown 240 apps that I've installed, alphabetically sorted from A-Z. I test a lot of apps for work and I've had this account since Day 1 with my ADP1(G1), so this list only makes it to the G's before it's full.
When I activated my Nexus 7 this week, I went into this list to start installing some of my favorite apps and I noticed a bunch of apps that I know I never installed. I know, you're thinking "how do you know, you say you install a bunch of apps". Well, I know I would never install some of these, even if it were part of my job. Not porn or anything, just not my style. For example, the controller for a nabaztag. I don't own one, so I've never needed the app. Out of those 240 listings, I can count at least 15 that I've not deliberately installed.
Partial listing here: http://twitpic.com/a9sw74 (900x4000px PNG)
So, immediate panic that someone compromised my Play account, but the question became, "why would they install a bunch of stupid apps that didn't cost anything when they could install apps that have monetary impact". I let my son and daughter mess with my phone or tablet in the restaurant, but they know not to install apps and none of those apps would appeal to a 14 or 9 year old. Regardless, I changed my password and revoked every device except the 3 I'm currently using.
I went into the "My Account" listing of all my purchases, which also includes free apps. Not one of these 15 apps that I've found show up in that list. Which makes me wonder if this is a glitch in Google Play as opposed to someone accessing my account incorrectly. Has anyone else experienced strangeness in this list?