This is a special scenario that occurs if:

• Reference count, d->ref > 1
• There is already more data allocated than the asize in current call of reserve

Code example:
1: QString A;
2: A.reserve(100);
3: A.fill('x', 100);
4: QString B = A;
5: A.reserve(50);

Instruction 2:
Entering reserve:
asize : 100
d->ref : 31
d->alloc : 0
d->capacity : 0

Exiting reserve:
asize : 100
d->ref : 1
d->alloc : 100
d->capacity : 1

Instruction 4:
d->ref becomes 2

Instruction 5:
Entering reserve:
d->alloc : 100
d->ref : 2
asize : 50
d->capacity : 1

Exiting reserve:
asize : 50
d->ref : 1
d->alloc : 50
d->capacity : 1

In this example allocation got truncated from 100 to 50, although previously 100 was reserved and the content of data block was filled with 'x'. As reserve should never truncate data, this is probably a bug.