Caro-Kann advanced variation pawn sacrifice I used to play the Caro-Kann a lot as black. One line that I reached many times in advanced variation is: 

   [FEN ""]
1. e4 c6 2. d4 d5 3. e5 Bf5 4. g4


In this position I saw one game where Anand played Bc8 and in my experience this line is very solid. The move Bg6 suffers some pressure from the pawn sacrifice e6, when the development of the bishop on f8 becomes difficult, and white easily controls e5. I saw one game Topalov vs Morozevich where Morozevich played Bg6, but then Topalov did not commit himself with e6.

My question is: should the pawn sacrifice e6 be feared?
The move 4. g4 is slightly dubious compared to the more solid option of 4. Nc3 (among Nc3), as it forces white to play 5. f3 if black opts for the mainline 4... Be4. Therefore I recommend this choice in general. It does, to an extent, weaken the idea of e6. However, 4... Bg6 is an option and a more common one at that.

The pawn sacrifice is actually quite common at the amateur level and can be an issue for the unprepared. Your most principled choice in your line is 5... fxe6 and it does work quite often well. After the mainline (if it can be so called) 6. Bd3, black should continue 6... Bxd3 7. Qxd3 Qd6 followed by the eventual fianchettoing of the bishop.

While this will not give you a simple win as black, it is clear that black's position is at least equal.