goggles leaking sometimes when swimming

If I have my swim cap pulled down too far in front I'll get a leak...it creates a small gap where the water can get in. 

I'm no expert but I may be able to add some info to the mix.  

If your goggles are leaking they likely don't fit your face for one reason or another - go to a local store with swim gear and get someone to show you how to pick a pair of goggles that fit.  Basically when pressed to your eyes they should suction cup to your face without falling off.  If they fall off they don't fit.  If the store salespeople don't know how to fit goggles to your face go somewhere else.  I bought my most recent goggles from Amazon but I knew what I wanted.  Don't buy from Amazon until you know what fits your face.

If they leak or are painful they aren't right for you.  If they leave their outline around your eyes they are too tight.

Swim caps, beards and scrunchy faces can cause leaks.  I don't wear swim caps except in races, I have a beard but I don't have trouble with my goggles - most of the time.

I started out with Aquasphere goggles and they worked well for me but in 2013 I was given a pair of Zoggs (I'm currently wearing a pair of Zoggs Predator Flex) and they have been great (for me).  They last well and I've never tightened them.  I opened the package and put them on and wear them as they come out of the package.  That is just lucky - not everyone can do that but some goggles should fit you like mine fit me.

Like Godawgs, I spit in my goggles, wipe the lenses with a fingertip to coat the lenses  then lightly rinse the goggles in the pool then put the goggles on, make sure I have no water in them and then press them to my eyes just enough to create that suction cup feeling then I start swimming and an hour later I take them off with no more than minimal discomfort (mostly none at all).

I hope this helps.

Check the nosepiece.  If they're adjustable, did the nosepiece slip one notch out?  

The goggles need to seal against your eye sockets, not against your temple.  Put them on, and look in the mirror.  Angle your head, and look to see if the goggles are trying to seal against your temple by moving your jaw.  If the temple movement is under your goggle seal, it'll leak.