If you play the line 1.d4 d5 2. c4 e6 3. Nf3 Nc6 4. Nc3 Nf6 5. Bg5 in the Explorer it will stop taking moves and freeze on white replies to ...Nf6.
It appears reversing the sequence of knight moves doesn't fix the problem either. It's not a big deal, but maybe something to look at eventually.
Thanks for spotting this issue. We will work to correct this as soon as possible.
I think I found another one:
1.d4 d5 2.Nf3 c5 3.c4 dxc4 4.dxc4 d5 5.a6
It doesn't show the a6 move. You can continue on after that, but the a6 move doesn't show up.