The Edit Capture Time command doesn't preserve fractional seconds when shifting the time of multiple photos. For example, if you select a batch of photos taken with "continuous shooting", e.g. at a sporting event, and then use Edit Capture Time to shift their time by 60 minutes, the fractional seconds of each photo will be dropped and the photos will no longer be correctly ordered by capture time.
To reproduce, take several photos that were all shot within the same second with the continuous-shooting option of the camera. LR will correctly transfer the capture time into their .xmp sidecars:
[XMP] Date/Time Original : 2011:05:13 18:32:44.51 [XMP] Date/Time Original : 2011:05:13 18:32:44.52 [XMP] Date/Time Original : 2011:05:13 18:32:44.53
Then select those photos and invoke Edit Capture Time. Use either the "Adjust to a specified date and time" or the "Shift by set number of hours" option to shift the time by 1 hour. Observe that the fractional seconds in the sidecars is now dropped:
[XMP] Date/Time Original : 2011:05:13 19:32:44 [XMP] Date/Time Original : 2011:05:13 19:32:44 [XMP] Date/Time Original : 2011:05:13 19:32:44
And the photos will no longer sort correctly by capture time in the Library.
This bug is easy to fix simply by doing the time adjustment with floating-point calculations, rather than integer.