There is a bug in current (revision #7268) sdcc that results in some array accesses going to the wrong lcoation if the local stack size exceeds the range reachable by the frame pointer in the z80 port.
Philipp Klaus Krause
Increasing priority since bad code is silently generated.
There now is an (#ifdefed) regression test for this issue at support/regression/tests/bug3482146.c.
Fixed in revision #7270.