practical wise:

#Multivalue :- this error will occur in 3ways
1) #multivalue in aggregation 
2) #multivalue in breaks header or footer
3) #multivalue in section level.

1:-- the o/p context not include i/p context its  situation
this error occurs.
Ex: in a report i have year,city dia's & Revenue measure is
there. 
         =<revenue> In <year> 
The above condition will to run the query getting revenue
column #multivalue error occurs.

solution: cilck the formulabar in view menu
         select the error containg cell, edit the formula to
write below condition.
= <revenue> In(<city>,<year>) In <year>
The above formula will run correct data will appear in the
report.
Note: the above condition by default it will take agg "sum"
function.

#syntax:-- 
          the variable in the formula no longer exist in the
 condition.

Ex:- <product price> * <quantity sold>
The above condition will run this error will occur.

Solution:- Click edit dataprovider--> new object will be
need --> select error cell --> edit formula --> click ok.

#error:--
         the variable in the formula is incorrect. this
error will occur.

solution : go for data menu --> click variable 
select the error containing a cell --> copy the formula in
edit menu --> paste it in new cell --> go for formula bar in
view menu -->  --> take the first error containg cell
-->edit the formula --> repeat the above steps.