The PrintGC/PrintGCDetails GC pause output during initial mark pauses can see interference from the PrintGC output of the concurrent mark thread. For example:

276.649: [GC pause (young) (initial-mark), 0.59312600 secs]
372.460: [GC pause (young)372.918 (initial-mark), 0.45774300 secs]
3210.137: [GC pause (young)3210.289 (initial-mark), 0.15259300 secs]
3681.914: [GC pause (young) (initial-mark), 0.27059100 secs]
3692.334: [GC pause (young)3692.501 (initial-mark), 0.16728700 secs]
3698.191: [GC pause (young) (initial-mark), 0.14413600 secs]
3705.374: [GC pause (young) (initial-mark), 0.16145600 secs

and:

276.649: [GC pause (young) (initial-mark), 0.59312600 secs]
   [Parallel Time: 575.5 ms]
277.242: [GC concurrent-mark-start]

372.460: [GC pause (young)372.918 (initial-mark), 0.45774300 secs]
: [GC concurrent-mark-start]

3681.914: [GC pause (young) (initial-mark), 0.27059100 secs]
3682.184: [GC concurrent-mark-start]

and:

3210.137: [GC pause (young)3210.289 (initial-mark), 0.15259300 secs]
: [GC concurrent-mark-start]

3698.191: [GC pause (young) (initial-mark), 0.14413600 secs]
3698.335: [Parallel Time: 137.2 ms]
[GC concurrent-mark-start]

Most of the time [Parallel Time: comes out as:
3699.247: [GC pause (young), 0.21151000 secs]
   [SATB Drain Time: 0.0 ms]
   [Parallel Time: 166.0 ms]

This seems to happen more frequently when the GC logging output is directed to a file using the -Xloggc:<file> flag rather than when it is directed to stdout.

The irregularity and randomness of the output makes it difficult to parse.