On numerous occasions, across all four browsers I've tried (Chrome, Firefox, Internet Explorer, Opera), this game has crashed and it virtually always does so in missions.
While simply scouring the problem would be nice, I've done enough programming to know the problem is not likely an easy one to solve and could be solved my any of multiple points in the code. However, there is a way to minimize the negative effects caused - while the time spent can never be gained back, and saving every N units of time in a mission so people can keep their items would almost certainly not be worth the effort of coding, at least being able to go back into that area would be one saving grace. I know the game records injuries even if it boots you out of a mission, you lose all items and materials and it locks out the area.
If the marker identifying the area as locked out was only tripped when the game successfully loads out of a mission, then this would skirt around the vast majority of crashing during a mission. If we can go back in and try again, then it's annoying instead of something that makes people curse and swear never to spend a dime on the game.