One example of "missing" line, based on one Watson's work, in Euwe/Be2 variation
1.e4 e6 2.d4 d5 3.e5 c5 4.c3 Nc6 5.Nf3 Bd7 6.Be2 Nge7 7.Na3! (in another thread on chesspub I suggest the loosing tempo 7.Bd3 but no good prospects for White) and now in book - volume 2, Encyclopedia Table 5, 160 sublines, only for Black 7...Nf5 and 7..Ng6.
The line starting with 7...cxd4 is completely overlooked.
And now 8.cxd4 Ng6!? 9.h4 (or 9.Nc2 h6!? Dom..because 9...Be7 10.Bd3 Nb4 11.Nxb4 Bxb4+ 12.Kf1 Be7 13.h4 is +=) and you reach a standard position where 9...Bb4+, 9...Bxa3, 9...Be7 are the common moves but I think Black can improve on games with 9..h6 (occured first time in Slevic-Nikolic,Celje 2003) 10.h5
with 10..Bb4 (instead of 10..Bxa3) 11.Kf1 Nf8 a position where Sveshnikov gives assessment as position is good for White forgetting a plan with Nh7-Ng5 given by Watson. Then 16.Nc2 Be7 17.Rh3 f5! because doesn't work 14.Rg3 Ng5 15.Nxg5 fxg5 16.f4 Qc7 (Watson Play The French 3Ed page 33; Movsevian-Nikolic, Istanbul 2000 analysis) 17.fxg5 f6 but now 18.Qd3!