Q:

Why are my objects changing unexpectedly?

I've got a very basic example where I'm reading some data into a class from JSON, and my objects are getting corrupted somehow. I suspect I'm missing some detail about how properties/ARC is working, but I'm not sure what it is, or how I'd track it down.
I've reduced my original code so that the problem is obvious. However this means that its not clear why I'm using custom properties etc. - my real code has more functionality in there...
The issue can be seen in the last line of the Test.m file. The first constructed object now contains data from the second object rather than the value it originally contained. 
Any advice on what I'm doing wrong and/or how to track down this kind of issue would be greatly appreciated.
ANBNote.h
@interface ANBNote : NSObject
@property (nonatomic,readwrite) NSArray* references;
- (id)initWithJson:(NSDictionary*)data;
@end

ANBNote.m
#import "ANBNote.h"

@implementation ANBNote
NSArray * _references;

-(id) init {
  if(!(self=[super init])) return nil;
  _references=@[];
  return self;
}

-(id)initWithJson:(NSDictionary *)jsonObject {      
  if(!(self = [self init] ) ) { return nil; }    
  _references = jsonObject[@"references"];    
  return self;
}

-(void) setReferences:(NSArray *)references {
  _references = references;
}

-(NSArray *)references {
  return _references;
}    

@end

Test.m
...
NSDictionary * d1 = @{@"references":@[@"r1",@"r2"]};
NSDictionary * d2 = @{@"references":@[@"q1",@"q2"]};

ANBNote * n1 = [[ANBNote alloc] initWithJson:d1];
NSLog(@"R1 = %p, %@", n1.references, n1.references); // Prints r1, r2 - as expected

ANBNote * n2 = [[ANBNote alloc] initWithJson:d2];
NSLog(@"R2 = %p, %@", n2.references, n2.references); // Prints q1, q2 - as expected
NSLog(@"R1 = %p, %@", n1.references, n1.references); // Prints q1, q2 - Oh No!

Note that if I remove custom references property functions and rely on the compiler generated version everything seems to behave correctly.

A:

This is not a ivar:
@implementation ANBNote
NSArray * _references;

it's a global. There's only one for all the instances of your class, not one for each. When the next instance sets it, the previous instances see the new values because it's the same variable. You need to put it into curly braces to make it an ivar:
@implementation ANBNote
{
    NSArray * _references;
}

There's no need to declare the variable explicitly, though -- you can still implement accessors yourself and let the compiler create the ivar as long as you use the default synthesized name (underscore + property name).

