The actions of a prison guard who mistakenly unlocked a section of cells and negligently relied on an inmate to voluntarily return to a cell unescorted did not rise to the level of deliberate indifference vis-a-vis the subsequent fisticuffs.
http://media.ca7.uscourts.gov/cgi-bin/rssExec.pl?Submit=Display&Path=Y2018/D07-10/C:17-1707:J:PerCuriam:aut:T:fnOp:N:2184277:S:0
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.