After a quick browse through the plugin API, I think I see a big potential for segfaults in plugins.
The Map functions:
unsigned char* (*getMapData_block)(int x, int z);
unsigned char* (*getMapData_meta) (int x, int z);
unsigned char* (*getMapData_skylight) (int x, int z);
unsigned char* (*getMapData_blocklight)(int x, int z);
seem to return pointers into the living data structures. Now suppose the plugin stores such a pointer and accesses it at a later time. But MineServer may have already released those chunks! BANG

Options:
1, Leave it as-is, no plugin ever will be so stupid (the ostrich approach)
2, Remove the routines altogether, replace with a single-block getter / setter function (will make some "terraforming" plugins painfully slow)
3, Upon call of these routines, increment a "lock" on the chunk; provide a lock-decrement function for the plugins to call when the chunks is no longer needed; don't release the chunk until it has a nonzero lock count (can cause memory starvation if plugins hold too many locks)
4,