Sh*t, as they say, happens. The benefit of experience is that you - generally - recognize it more quickly when it does and know how to best fix it.
I haven't used a 12 dent reed for a while and it's surprising how much finer it is than, say, 8. I caught myself a few times loading a dent with either too many or two few ends, but obviously missed this one. Until I started to tie on.
At first I thought I'd skipped a dent entirely, determined that it was closer to the right selvedge than the left, sighed and carried on tieing up. Because it's easier to move everything over one dent if the part that doesn't need fixing is already secured.
Closer inspection revealed what had actually happened. Two ends got caught and dragged into an adjacent space, so nothing actually needs to be fixed except the dent that has four instead of 2 and the space that is empty and needs two.