What we are talking about here is a serious behavior change how statements like...
insert /*+ append */ into t values (1)...are handled in 11G.
Saving yourself from yourself... not anymore
In previous Oracle versions, the append hint in the cases like above was simply ignored for some good reasons. The hint is being obeyed in 11G.
To better elaborate what that means, let's take the following example:
begin...and compare how behavior will change in 11G.
for i in 1 .. 1000
insert /*+ append */ into t values (i);
First of all, I've seen legacy code like the above in many places where developers were putting append hint everywhere they could because they heard it is like fast=true thing.
After you upgrade, you might expect the following:
If you don't do commit after each row -- the code will break itself with ORA-12838: cannot read/modify an object after modifying it in parallel.
If you do commit (as in my example) -- you have all chances to grow the table beyond the skies as each direct path insert writes beyond the HWM (in 11G, the above code will grow the table by 1000 blocks each time it is executed).
Serious locking issues as each direct path insert has to obtain an exclusive lock on the table first.
The above might produce quite an unpleasant surprises for some of you...