Some time ago I took part in a debate about the problems of a slipping anchor in Spey casting. One particular instructor stated that, apart from hooking the vegetation behind you, a slipping anchor shouldn’t be a concern.
I don’t agree. Apart from hooking a branch, if the anchor slips in a pretty “clean” scenario there is still the possibility of breaking the hook point against some stone on the bank —probably a worse thing than hooking a tree, as the damaged hook will remained unnoticed.
Moreover, if the line moves backward during the delivery stroke some of the energy needed to send the fly to its target is wasted in that motion, as explained here.
The video opening this piece shows the result of a badly placed anchor combined with a sweep with too steep an incline: the line slips backward and the delivery loop “loses steam” before unrolling.