Note: Make sure you get the axle shaft in the trans ALL the way.
I don't know how much of a difference there is between auto and manual trans, but when I replaced mine (auto trans) I didn't quite get it in ALL the way. As I was cruising down the road it popped out. My car wouldn't move and I thought my trans went out.

 

A manual transmission will do the same thing if the axle isn't all the way locked in. Happens a lot, even on professional repairs.

 

You sayin im not a professional
Yeah I didn't realize that it actually POPed in. I kept hitting it in and heard no noise so I figured it was in. Guess I was wrong, stranded in the middle of no where with no cell phone 30 miles away from my house haha.
Anyways, make sure you hear it click or pop in.

 

Or try lightly to pry it out again, if it slides right out it isn't latched properly.

 

mk378 - I heard from somewhere that if the boot is torn, you can use a lighter to heat up the rubber and re seal it. Is this true?

 

Absolutely not true!

 

^ but you can spend 25$ for a boot replacement and it comes with new grease

 

Ok, lets look at this logically.

You have to remove the axle to re-boot it.
99.9% of the time by the time you notice a torn boot you have driven it for many miles and water has gotten in and if it isn't rattling/clicking now - trust me it will
The price of a remanufactured axle is $50-$70 (unless you have an exotic or rare car) and they usually come with a LIFETIME WARRANTY (so if the boot rips again = new axle for free)
So the best thing to do obviously is to install a reman axle FTW!

 

^ i know that my respose was about using the lighter.... if you tear it while installing it or it just happens then you can save 25-40$ and just buy the boot... if its been torn for awhile and you just notice then yea buy the cv axle ... like my sedan it had a small tear in the inner boot .. im trading the car im not spend more than what i have to.. so 25$ is better than 65$. but i did pay 65$ for the other side to replace the whole cvaxle because it was shot.