Delayed extents is probably a normal thing that happens in daily I/O operations. The possibility of it not actually hitting the platters permanently is a scary scenario.
"A single extent in ext4 can map up to 128 MiB of contiguous space with a 4 KiB block size."
The whole bug sounds similar to a problem we had on XFS back in the 2.6.16 time frame. Certain operations were delayed for hours meaning that it was possible for renames/writes not to hit the platter if the machine experienced power loss instead of a clean shutdown, even if sync's were performed.
"A single extent in ext4 can map up to 128 MiB of contiguous space with a 4 KiB block size."