This is an old original post, but I have come across it many times and from the span above its somewhere people land a lot. Its also still a problem if following that old 12 year old tutorial (3ds Max 2026). Someone above posted, don't mirror bones, and I agree. The way I do it is as follows:
I first create point helpers for all the limb joints on one side, plus the final end points - shoulder, elbow, wrist, hand, base of each finger, one for each knuckle and the finger tips; top of the thigh, knee, ankle, midfoot, ball of foot, each toe knuckle and tips of each toe. Leaving Nubs on.
Then I hide all but those helpers. Switch the the view that makes most sense. Turn on snapping Pivot/End Point flags only. Then create the bones by connecting the helpers from shoulder to finger tips and thigh top to toe tip.
Then is check the X position of the helper nearest the x axis (lowest x value) - usually the shoulder and make a note of it. With helpers set in the selection filter (ribbon), I then select all the helpers and mirror them about X with an offset equal to twice the x value I noted down.
Then I hide all the original helpers, leaving the mirrored ones, and create bones for them in the same way.
Leave the NUBs on.
Then select all the new limb bones, except the NUBs, and rotate the pivot points 180 degrees in Y.
Select all the new bones and in bone tools click the Realign, reset stretch and reset scale buttons. (its a good ides to do this to all bones if the pivot is changed).
Not as fast a just mirroring, but not that much slower - and without all bugs and stress of fixing things after with crazy workarounds.