It's often a funny part in the movie.
The couple have just said their vows and the officiant asks for the rings. The best man pats down his suit, looks shocked and confused when he realises he doesn't have them. Everyone scurries around looking until a substitute (like a coke can tab or cheezel) is found last minute to replace the ring. Everyone is received and go on to live happily ever after.
Yes, it is funny losing a wedding ring in a movie but not so in real life. Looking after wedding rings, until the moment you cement your vows with a ring, is a big responsibility. It can cause some couples to worry about what they should do. They ask what is the correct way and what do other people do?
Q: Who should look after your rings?
A: Anyone you wish too!