incr() returns -1

Hi,

I am using incr() of couchbase client to increase a counter key in a bucket. I found this method very useful in some of our usecases. Around 150 threads from four machines increments current value of counter by 10 and use the values for some internal logic. Initially everything worked as expected. However a few days back I realized that couchbase returns -1 for some of the incr() method calls. That is how I noticed that incr() in MemcachedClient could return -1 if there is a failure to update the counter:
http://www.couchbase.com/autodocs/couchbase-java-client-1.4.3/net/spy/memcached/MemcachedClient.html

I have a few queries about this return value:
1. Could you please tell me the cases in which incr() would return -1? I identified one case. If the RAM quota allocated to the bucket is exhausted, we are likely to get -1. During this time addition of key-value pairs into the same bucket using add() will return false through the OperationFuture object. What are the other cases that would return -1 in the incr()?

2. Could you please tell me how the following method call works?
http://www.couchbase.com/autodocs/couchbase-java-client-1.4.3/net/spy/memcached/MemcachedClient.html#incr%28java.lang.String,%20long,%20long,%20int%29

I would like to know when the default value would be returned and when -1 would be returned. I think the default value would be returned if it is possible to insert the value into couchbase and -1 would be returned if it is not possible to retrieve the counter and update it. Am I correct?

3. Till now my project did not have a use case to increment negative values. However if somebody wants to increment -2 by 1 then the result would be -1. How will the client call be able to distinguish failure and actual increment in this case? Wouldn't it be better to throw a checked exception from incr() than returning -1? I think with checked exception we can provide more information about the cause of failure.

Richards Peter.