BasePortalVis:
(3.68 seconds)
Warning: Leaf portals saw into leaf
Problem at portal between leaves 305 and 310:
(2482.999 -875.001 51.000)
(2482.189 -875.378 51.000)
(2473.000 -869.000 60.000)
(2749.000 -740.200 60.000)
(2749.000 -742.000 58.481)


what does this mean?

Well this means that you have a leaf that isn't convex. David Hyde once gave the analogy 'It's equivalent to looking out the window of a room and seeing back into the room' and this pretty much explains it.

What can you do?
This can be a very difficult problem to solve as you can't track it down normally as it more to due with portals than anything else.You basically have an over complex area thats generating this warning(check cyclindrical and vertex manipulated brushes first) and you can either simplify it, delete it or just turn it into a func_wall.

source (http://members.lycos.co.uk/EditHalfLife/tutorials/leafs.htm)

Did a quick Googlesearch ;)

succes

This error can also be 'caused if you have applied the 'clip' texture to an entity (such as a func_wall.) To fix, simply change the entity back to a brush or change the texture.

Pretty sure if you put clip on an entity it'll cause much more problems than a leaf into leaf problem. Often a leaf into leaf can be ignored...but occasionally it'll make a funky error that would often be quite visible.