badblocks stuck in a loop?

I'm using a program called badblocks to scan disks for errors, and I'm finding that it sometimes appear to get stuck in a loop. I'm using in read/write mode and by default it tests four patterns. Here's an example:

-w -s /dev/sdf -b 4096

-w -s /dev/sdf -b 4096
Testing with pattern 0xaa: done                                                 
Reading and comparing: done                                                 
Testing with pattern 0x55: done                                                 
Reading and comparing: done                                                 
Testing with pattern 0xff: done                                                 
Reading and comparing: done                                                 
Testing with pattern 0x00: done                                                 
Reading and comparing: 1.79% done, 23:39:29 elapsed. (0/0/0 errors)

Okay, looks good, except that this is now the third time it's done the final pattern (0x00). When it reaches 100% it just resets the % counter and starts over. This job is not currently running, but I'm now running bad blocks on another machine with another disk, and I've noticed that it's repeating the second pattern. Any ideas? In both cases no errors are reported (so far).