I have gotten the following errors using ruby 1.9.2-p320 with Heroku's Taps-using 'db' commands such as:
heroku db:pull --app my-app
Which produces errors like:
Taps Load Error: no such file to load -- sqlite3
-or-
Taps Load Error: no such file to load -- pg
(And yes, btw, I have all the correct gems, including both postgres--running fine locally--and sqlite3, as well as having uninstalled, re-installed, and updated the taps & heroku gems and including them as well as not including them in my Gemfile. I'm running Rails 3.2.6)
Although there is a known bug that results in a "time zone displacement" error: heroku db:push not working, using taps gem, I have not seen any SO posts that directly address this issue.
The solution I found, using hints from https://github.com/ricardochimal/taps/issues/92 is detailed here: https://github.com/ricardochimal/taps/issues/92#issuecomment-6486535
The short of it, is that ruby 1.9.2-p320 seems to have been the problem, as far as I can tell. Downgrading to ruby 1.9.2-p290 fixed it for me. This might be related to the ruby 1.9.3 issues spoken about at the above link, not sure.
If you are still hanging in there with Taps, please see this comment at GH, as the poster doshea seems to have looked into it even more thoroughly than I or jfeust have.
I'll also reiterate here that if you don't need Taps, then heroku-pgtransfer is your friend. See Ryan's answer to my question here and his excellent blog post on using the newer tool.