It looks like former Ohio State wide receiver Brian Hartline was at Buckeyes' practice today.
The seven-year NFL vet apparently participated in the session, helping his alma mater prepare for Clemson. This is perfectly legal, and judging by this tweet from OSU quarterback Dwayne Haskins, Hartline showed he can still play.
Hartline responded to the tweet, saying "I'll be back tomorrow with a little less rust haha good work today." That response has since been deleted, but CollegeFootballTalk has captured a screenshot here.
Why did he delete the tweet? Well, there's a chance it might have been an NCAA violation.
According to NCAA bylaw 18.104.22.168, "a former student at the certifying institution (e.g., former student-athlete) may participate in an organized practice session on an occasional basis, provided the institution does not publicize the participation of the former student at any time before the practice session." Hartline probably deleted the tweet to cover OSU against anyone saying they "promoted" him being there tomorrow.
However, is Hartline tweeting about it really the same as Ohio State doing it? Either way, if this is deemed to be a violation, it surely is a very minor one and will be met with a slap on the wrist, at most.