How could I get "String or binary data would be truncated" when the value being updated is not too long?

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SQL

T-SQL

stored-procedure

On calling a Stored Proc to retrieve values, in some cases (not all - it all works fine with some data), I get a "String or binary data would be truncated" err msg .

According to what I've read, this exception is thrown when you try to insert data that is too long, or when you try to add data out-of-order; the latter can't be the problem, because it does work in some cases. It's apprently a data problem.

The exeption message says line 75 of "priceUsageVariance" (my Stored Procedure) is the culprit:

Line 75 of "priceUsageVariance" is:

WHERE ItemCode='X'

Here is an excerpt from that Stored Procedure, to show more context (the ostensibly problematic line is the last one):

. . .
CREATE TABLE #TEMPCOMBINED(
  PlatypusNo VARCHAR(6),
  PlatypusName VARCHAR(50),
  ItemCode VARCHAR(15),
  PlatypusItemCode VARCHAR(20),
  DuckbillDESCRIPTION VARCHAR(50),
  PlatypusDESCRIPTION VARCHAR(200),
  WEEK1USAGE DECIMAL(18,2),
  WEEK2USAGE DECIMAL(18,2),
  USAGEVARIANCE AS WEEK2USAGE - WEEK1USAGE,
  WEEK1PRICE DECIMAL(18,2),
  WEEK2PRICE DECIMAL(18,2),
  PRICEVARIANCE AS WEEK2PRICE - WEEK1PRICE,
  PRICEVARIANCEPERCENTAGE AS CAST((WEEK2PRICE - WEEK1PRICE) / NULLIF(WEEK1PRICE,0) AS DECIMAL(18,5))
);

INSERT INTO #TEMPCOMBINED (PlatypusNo, PlatypusName, ItemCode, PlatypusItemCode, DuckbillDESCRIPTION, PlatypusDESCRIPTION,
WEEK1USAGE, WEEK2USAGE, WEEK1PRICE, WEEK2PRICE)
SELECT T1.PlatypusNo, T1.PlatypusName, 'X', T1.PlatypusITEMCODE, NULL, T1.DESCRIPTION, T1.WEEK1USAGE, T2.WEEK2USAGE,
T1.WEEK1PRICE, T2.WEEK2PRICE
FROM #TEMP1 T1
LEFT JOIN #TEMP2 T2 ON T1.PlatypusITEMCODE = T2.PlatypusITEMCODE

    UPDATE #TEMPCOMBINED SET ItemCode = ISNULL(
    (SELECT TOP 1 ItemCode
     FROM MasterPlatypusUnitMapping
     WHERE Unit=@Unit
     AND PlatypusNo=#TEMPCOMBINED.PlatypusNo
     AND PlatypusItemCode = #TEMPCOMBINED.PlatypusItemCode
     AND ItemCode IN (SELECT ItemCode FROM UnitProducts WHERE Unit=@Unit)),'X'
    )
    WHERE ItemCode='X'
. . .

I don't see how this problem is even possible - The ItemCode field is either being updated with an ItemCode value from the MasterPlatypusUnitMapping table - which is a VarChar(15), the same as the corresponding field in my #TEMPCOMBINE table - or with an 'X'. How could either value be too large?

Is the line number given valid/reliable? Is there a way to step through the Stored Procedure as it's being processed?

Is there some kind of workaround so that I can prevent this exception from fouling up the works?