Or unless they get more districts that what belongs to them by PR, although this doesnīt happen so often.
Actually, it happens quite a lot, especially since 1990, that CDU, and SPD get more districts than their total of seats allocated in PR. The solution is to extend the number of seats in the Bundestag, socalled "Uberhangmandate" (overhang seats).
There is also a way to avoid the change in the number of seats (I honestly donīt remember how). Itīs useful in cases where (e.g., bc the constitution doesnīt allow it) you canīt change the number of seats.
This system has it all...
The only way how you can avoid changing the no of seats would be not to award seats to the narrowest winners of that party - this is actually done in Bavaria, though it only ever happened once there.
What's done in most German states is to give additional seats to all the other parties as well (Ausgleichsmandate), so that representation remains strictly proportional.
However, that can't be done in the federal elections because you'd have to adjust between the states as well as between the parties. Now, if the SPD wins all 10 direct seats in, say, Sachsen-Anhalt (as they did in 2002), but should only have 7 (8 in 2002, but 7 wouldn't be impossible if the PDS has a good year), you'd have to expand parliament by almost 50%!
So, in federal elections, nothing of the kind is done.