Chunks for new exceptions are allocated sequentially at the time when they are first accessed. The are committed to on-disk exception table when their copying finishes. Because that copying can finish out of order (the block layer or disk may reorder requests), exceptions are sometimes written out-of-order into on-disk exception storage.

Normally there is no problem with out-of-order exceptions, the exception record contains both new and old chunk, so they will be loaded correctly on next activation and there is no data corruption.

However, when the computer crashes at certain point (when the high-chunk number exception record is written but succeeding low-chunk number exception is not yet written), the space for that low-chunk number exception record is lost and will never be used.

The result is that snapshots with sufficient space can overflow because of this behaviour.

Example (assume 4k chunk size):
- write simultaneously to chunks 0 ... 511 on the origin.
- exceptions are allocated at places 2 ... 257 and 259 ... 514 in the chunk storage. All the exceptions are being copied simultaneously
- now assume that copying of exceptions at origin 256 ... 511 (snapshot 259 ... 514) finishes first
- records containing origin 256...511 and snapshot 259...514 are written into exception metadata area at chunk 1

now assume *CRASH*

after reboot, 256 exceptions from the metadata area at chunk 1 is correctly loaded. The next free snapshot exception number is set to 515. The snapshot chunks 2 ... 257 are lost and can't ever be used.