Pro tip: some classes you may not be able to get credit for. If this happens, you could be stuck unable to take essential courses without doing summer school or graduating late. Consequently, you want to look as early as possible into the offerings of the school you are interested in so that you can take classes before traveling if you will need them afterwards and can't get an equivalent abroad.
Good Example: Engineering and Design I is a class that CMU does not accept transfer credit for. Although students usually take it Junior year, my adviser bumped me into the class as a sophomore.
Bad Example: a previous MechE studying at RWTH took a masters course there to count for Dynamic Systems and Controls (DSC) here. When I tried to get the same exact course approved this semester, the professor in charge of the decision had changed. The new professor not only took months to give a reply, but he did not accept the course or any similar ones. While I waited for his reply, I operated under the assumption that the credit would transfer, did not take DSC this semester, found out that I could not take it next year, and was stuck with the option of trying to take the class that DSC is a pre-requisite for in tandem with it or taking an extra semester to graduate. I went for the last option and decided to throw in some graduate courses to get a master degree while I was at it.
Another pain in the butt is communicating with an adviser in Aachen who lives ~7 hours off of my schedule and takes a long time to reply to emails. This, I have heard, is typical for Europeans, who are frequently out of the office and do not take work with them on vacation. I'm sure I'll appreciate that more when I take a German vacation. Even so, be sure to build good relationships with any contacts you have and ask them questions. You are not pestering them. It is an adviser's job to "advise."
That's all I've got for now. Use your resources, and plan ahead!