Daniel,
After your low probability hint I did some trouble shooting to root out the source of the error and I am honestly stumped. Hoping you guys can help me figure it out.

Again, just to repeat the problem, for my group-level analyses, I am running ttests using 3dttest++ with the -Clustsim flag for correcting for multiple comparisons. For a subset of my results, I am getting voxels with a z-score of 13. I understand now that AFNI is just maxing out the z-score, so I am trying to figure out why I am getting these voxels with such a high z-score. Here's what I've done so far:

1. I thought perhaps the issue was that there was only 1 data point for the voxels where I am getting z=13 causing some kind of error when computing the standard deviation or standard error and leading to these maxed out z-scores. Double checked and there are 40 datapoints going into the analysis for these voxels (and for my study N=40).

2. I thought perhaps for one subject the data at this voxel might just be an extreme outlier produced by some kind of error, thus leading to an extreme mean and an extreme z-score. However, I manually inspected all of the individual subjects values and they are all reasonable. Moreovoer, the mean value across subjects for these voxels (where z=13) is comparable to the mean of surrounding voxels (and at times even slightly lower) that have reasonable z-scores. For example the mean value for a z=13 voxel is 13.39, the mean value for an adjacent voxel is 16.62 but the z-score for that voxel is only 8.08.

3. This made me think that something was going wrong in the computation of the z-scores themselves. So I re-ran 3dttest++ without the -Clustsim flag to get t-stats. As far as I can tell, the t-stats are normal. At the voxel referenced above where z=13 and the mean is 13.39, the t-stat is 10.65. At the adjacent voxel, where the mean value is 16.62, the t-stat is 10.69.

All of this would suggest that my crazy z-scores are coming at some point during the cluster correction, when converting the t-scores to z-scores. But for the life, of me, I can't figure out what I am doing wrong. To make things more confounding, the vast majority of my analyses using this same script aren't producing these wild z-scores.

I would really appreciate any thoughts or ideas you have and thank you so much for your help!
Regan