I got the assert (very very infrequently) on a Solaris/x64 as well as a Solaris/sparc Niagara box when running GCBasher as follows:-

-d64 -server -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:+VerifyBeforeGC -XX:+VerifyAfterGC -XX:+VerifyDuringGC -Xmx64m -XX:+PrintGCTimeStamps -XX:+PrintGCDetails -XX:+ShowMessageBoxOnError -XX:+PrintCompilation -XX:CICompilerCount=1 -XX:+UseCompressedOops -Xbatch -jar GCBasher.jar -time:300000


170.266: [GC pause (young)170.450: (initial-mark), 0.18410294 secs]
   [GC concurrent-mark-start]
[Parallel Time: 172.8 ms]
      [Update RS (Start) (ms): 170278.1 170273.1 170272.6 170272.8 170273.4 170272.6 170279.1 170272.7 170272.6 170272.5 170272.8 170273.1 170272.6 170272.6 170272.7 170272.5 170272.9 170272.7 170272.7 170272.6 170272.8 170272.5 170272.6]
      [Update RS (ms): 27.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
       Avg: 1.2, Min: 0.0, Max: 27.8]
         [Processed Buffers : 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
          Sum: 1, Avg: 0, Min: 0, Max: 1]
      [Ext Root Scanning (ms): 6.0 1.3 1.0 0.9 1.1 0.9 7.4 0.8 0.7 0.6 0.8 1.0 0.6 0.5 0.5 0.4 0.6 0.4 0.3 0.3 0.3 0.1 0.1
       Avg: 1.2, Min: 0.1, Max: 7.4]
      [Mark Stack Scanning (ms): 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
       Avg: 0.0, Min: 0.0, Max: 0.0]
      [Scan-Only Scanning (ms): 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
       Avg: 0.0, Min: 0.0, Max: 0.0]
         [Scan-Only Regions : 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
          Sum: 0, Avg: 0, Min: 0, Max: 0]
      [Scan RS (ms): 19.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
       Avg: 0.8, Min: 0.0, Max: 19.4]
      [Object Copy (ms): 118.2 137.2 137.2 137.2 133.0 137.1 132.5 136.9 137.0 137.3 137.0 136.5 137.0 137.5 136.8 137.1 136.7 137.1 136.9 137.4 136.7 137.1 137.1
       Avg: 135.9, Min: 118.2, Max: 137.5]
      [Termination (ms): 0.0 33.0 33.2 33.3 37.3 33.3 31.3 33.6 33.4 33.2 33.2 33.6 33.5 32.9 33.6 33.4 33.4 33.3 33.5 32.9 33.6 33.4 33.3
       Avg: 32.0, Min: 0.0, Max: 37.3]
      [Other: 1.7 ms]
   [Clear CT: 1.3 ms]
   [Cur Clear CC: 0.0 ms]
   [Cum Clear CC: 0.1 ms]
   [Min Clear CC: 0.0 ms]
   [Max Clear CC: 0.0 ms]
   [Avg Clear CC: 0.0 ms]
   [Other: 10.0 ms]
   [ 64M->45M(256M)]
 [Times: user=3.91 sys=0.03, real=0.20 secs]
170.760: [GC concurrent-mark-end, 0.1081481 sec]
170.760: [GC remark, 0.0090474 secs]
 [Times: user=0.07 sys=0.01, real=0.01 secs]
170.770: [GC concurrent-count-start]
170.822: [GC concurrent-count-end, 0.0512969]
171.080: [GC cleanup 63M->29M(256M), 0.0069773 secs]
 [Times: user=0.02 sys=0.00, real=0.01 secs]
171.087: [GC concurrent-cleanup-start]
171.093: [GC concurrent-cleanup-end, 0.0053815]
# To suppress the following error report, specify this argument
# after -XX: or in .hotspotrc: SuppressErrorAt=/concurrentG1Refine.cpp:273
==============================================================================
Unexpected Error
------------------------------------------------------------------------------
Internal Error at concurrentG1Refine.cpp:273, pid=12698, tid=48
Error: assert(!is_young_card(cached_ptr),"shouldn't get a card in young region")

Do you want to debug the problem?

To debug, run 'dbx - 12698'; then switch to thread 48
Enter 'yes' to launch dbx automatically (PATH must include dbx)
Otherwise, press RETURN to abort...
==============================================================================


  [7] ConcurrentG1Refine::cache_insert(this = ???, card_ptr = ???, defer = ???) (optimized), at 0xffffffff7c51a290 (line ~273) in "concurrentG1Refine.cpp"
  [8] HRInto_G1RemSet::concurrentRefineOneCard(this = ???, card_ptr = ???, worker_i = ???) (optimized), at 0xffffffff7c6deb2c (line ~809) in "g1RemSet.cpp"
  [9] RefineCardTableEntryClosure::do_card_ptr(this = ???, card_ptr = ???, worker_i = ???) (optimized), at 0xffffffff7c6b9360 (line ~59) in "g1CollectedHeap.cpp"
  [10] DirtyCardQueue::apply_closure_to_buffer(cl = ???, buf = ???, index = ???, sz = ???, consume = ???, worker_i = ???) (optimized), at 0xffffffff7c624964 (line ~54) in "dirtyCardQueue.cpp"
  [11] DirtyCardQueueSet::apply_closure_to_completed_buffer_helper(this = ???, worker_i = ???, nd = ???) (optimized), at 0xffffffff7c625254 (line ~182) in "dirtyCardQueue.cpp"
  [12] DirtyCardQueueSet::apply_closure_to_completed_buffer(this = ???, worker_i = ???, stop_at = ???, during_pause = ???) (optimized), at 0xffffffff7c625324 (line ~207) in "dirtyCardQueue.cpp"
  [13] ConcurrentG1RefineThread::run(this = ???) (optimized), at 0xffffffff7c51b634 (line ~115) in "concurrentG1RefineThread.cpp"
  [14] java_start(thread_addr = ???) (optimized), at 0xffffffff7cee19c0 (line ~1019) in "os_solaris.cpp"