That was just about the final straw for Humphreys at the end of a week in which he saw the Lions selectors overlook him for this summer's South African tour and select Williams, Wales's third-choice hooker, instead.
Humphreys had said that his head-to-head duel with Williams in this match would be nothing personal. It was hard to believe that given the way the Welsh skipper clattered into his opposite number.
A number of other Cardiff players also seemed to want to take it out on Williams for getting the nod ahead of their man. Derwyn Jones in particular seemed to be carrying out a personal vendetta against anyone in a black shirt.
After their appalling performance earlier in the week against Newport, Cardiff were more sprightly in this game.
Their outside-half Lee Jarvis made good use of the wind in the first half as he landed five penalties and converted two tries for a 19-point tally in the opening 40 minutes.
The first try came after a quarter of an hour and was wholly attributable to the handling skills of Humpheys. Appearing on the wing he first took a long pass, then rode a tackle and finally sent his superbly delayed and controlled pass to Nigel Walker, who raced the length of the 22 to score.
A brilliant touchline burst by Cardiff's new Lion Robert Howley paved the way for their second try, this time bagged by Simon Hill at centre.
Cardiff's immediate target was to get three tries ahead of their opponents to secure a bonus point and when Hill raced clear for his second try it looked as though they could go on to get another. But the home side almost threw their bonus chances away as twice they let Neath sneak over for tries. Chris Bridges got the first, to which Steve Ford replied immediately, and then the Wales back-row man Steve Williams muscled through.
Cardiff's blushes were saved in the final move of the match when Jarvis broke clear and sent Jamie Ringer racing to the posts for the try which re-established their bonus point and keeps them in the top four of the First Division table.
Cardiff: J Thomas; S Ford, L Davies, S Hill, N Walker; L Jarvis, R Howley (J Hewlet, 57); A Lewis, J Humphreys (capt) (P Young, 66), L Mustoe, K Stewart, D Jones (L Jones, 52), M Bennett, O Williams, J Ringer.
Neath: D Case; C Higgs, D Hawkins, J Funnell, R Jones; Darren Morris (G Davies, 43), C Bridges; D Morris, B Williams, J Davies (capt) (L Gerrard, 56), S Martin, N Watkins, Robin Jones (S Gardner, 66), S Williams, I Boobyer (G Newman, 62).
Referee: R G Davies (Dunvant).Reuse content