Step 1:
    * Input some strings to each cells A1 - A10.
    * Input value 5 to the cell B1.
    * Input formula =ISBLANK(B1) to the cell B2.
    * Input formula =OFFSET(A1,B1-1,0) to cell B3.
    * Input formula =IF(ISBLANK(B1),"?",OFFSET(A1,B1-1,0)) to the cell B4.
  Results of each formula is....
    * B2: FALSE
    * B3: a copy string from A5
    * B4: a copy string from A5
  If you change the value of B1 from 1 to 10, you can see changing string in B3 and B4.
  B4 should indicate same value of B3. Because they refer same value from column A.
  It is ok.

Step 2:
    * Clear the value of the cell B1.
  Results of each formula is....
    * B2: TRUE
    * B3: Error:502
    * B4: ?
  It is ok.

Step 3:
  This step has probrems. I doubt this is bug of LibO.
    * Input value 5 to cell B1 again.
  I expect it return same results of Step 1. But it does not.
    * B2: FALSE
    * B3: a copy string from A5
    * B4: ?
  If you change the value of B1 from 1 to 10, B4 still indicate '?' anyway.
  It is not good.

I tried same steps in another software. OOo is ok. MS Office is ok either.
But LibO is not good.

Enviroment:
    * Windows XP Pro SP3, Windows 7 Pro SP1(x86), LibO 3.3.1 Japanese

ref: http://en.libreofficeforum.org/node/372