[Freesurfer] correcting deffect

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Hello FreeSurfer Team i have problem when run a data set, i re-run it but that error remain (just "convex hull" changed from 6148 to 6147!!)

this is the last line in terminal (that process doesn't exceed that after about 10 hours!!!)

"CORRECTING DEFECT 0 (vertices=21958, convex hull=6147, v0=0)"

but the first time i run this data set, there was more another line in terminal:

"XL defect detected ..."

now i want to know why this error occurred ? and what should i note in my data to never encounter this error again?

(I read some similar mails, but i didn't understand my questions)

thanks a lot again for your attention