First off, if you have it as tight as you say, it should not fly off. If it does, the usual suspect is that the engine is running lean - which will cause a backfire - which will throw a prop no matter how tight it is.

Sometimes a prop can come off if you have extra washers holding it on. The washers slip like bearings no matter how tight you have the nut.

Thread locking compound is not recommended and shouldn't be needed.

Secondly, NEVER stand in front of the engine once it is running! ESPECIALLY (and I can't emphasize this enough) if the engine is running up above an idle!!!

I can't believe how many seasoned fliers I have seen that will stand in front of their airplane and bring the throttle to full-bore!

I am one of those people who disregards a lot of safety rules (Like Red Green says, "Safety, because we have to" ) But common sense should tell you that if that prop is going to come off, it's not going to go BEHIND the engine - and THAT'S where you (and everyone in the area) should be!