Sporadically getting error "string or binary data would be truncated" in SQL server 2008 while inserting in a Table Type object

Question

I am facing a strange SQL exception:-
The code flow is like this:
.Net 4.0 --> Entity Framework --> SQL 2008 ( StoredProc --> Function {Exception})
In the SQL Table-Valued Function, I am selecting a column (nvarchar(50)) from an existing table and (after some filtration using inner joins and where clauses) inserting the values in a Table Type Object having a column (nvarchar(50))
This flow was working fine in SQL 2008 but now all of sudden the Insert into @TableType is throwing  "string or binary data would be truncated"  exception. 
Insert Into @ObjTableType
Select * From dbo.Table
The max length of data in the source column is 24 but even then the insert statement into nvarchar temp column is failing.
Moreover, the same issue started coming up few weeks back and I was unable to find the root cause, but back then it started working properly after few hours
(issue reported at 10 AM EST and was automatically resolved post 8 PM EST). No refresh activity was performed on the database.
This time however the issue is still coming up (even after 2 days) but is not coming up in every scenario. The data set, for which the error is thrown, is valid and every value in the function is fetched from existing tables. 
Due to its sporadic nature, I am unable to recreate it now :( , but still unable to determine why it started coming up or how can i prevent such things to happen again.
It is difficult to even explain the weirdness of this bug but any help or guidance in finding the root cause will be very helpful.
I also Tried by using nvarchar(max) in the table type object but it didn't work.
Here is a code similar to the function which I am using:
BEGIN
TRAN
DECLARE @PID
int = 483
DECLARE @retExcludables
TABLE
    PID
int NOT
NULL,
    ENumber
nvarchar(50)
NOT NULL,
    CNumber
nvarchar(50)
NOT NULL,
    AId
uniqueidentifier NOT
NULL
declare @PSCount int;
select @PSCount =
count('x')
from tblProjSur ps
where ps.PID
= @PID;
if (@PSCount = 0)
begin
return;
end;
declare @ExcludableTempValue table (
        PID
int,
        ENumber
nvarchar(max),
        CNumber
nvarchar(max),
        AId
uniqueidentifier,
        SIds
int,
        SCSymb
nvarchar(10),
        SurCSymb
nvarchar(10)
with SurCSymbs as (
select ps.PID,
               ps.SIds,              
               csl.CSymb
from tblProjSur ps
            right
outer join tblProjSurCSymb pscs
on pscs.tblProjSurId
= ps.tblProjSurId
inner join CSymbLookup csl
on csl.CSymbId
= pscs.CSymbId 
where ps.PID
= @PID
    AssignedValues
as (
select psr.PID,
               psr.ENumber,
               psr.CNumber,
               psmd.MetaDataValue
as ClaimSymbol,
               psau.UserId
as AId,
               psus.SIds
from PSRow psr
inner join PSMetadata psmd
on psmd.PSRowId
= psr.SampleRowId
inner join MetaDataLookup mdl
on mdl.MetaDataId
= psmd.MetaDataId
inner join PSAUser psau
on psau.PSRowId
= psr.SampleRowId
            inner
join PSUserSur psus
on psus.SampleAssignedUserId
= psau.ProjectSampleUserId
where psr.PID
= @PID
and mdl.MetaDataCommonName
= 'CorrectValue'
and psus.SIds
in (select
distinct SIds from SurCSymbs)         
    FullDetails
as (
select asurv.PID,
Convert(NVarchar(50),asurv.ENumber)
as ENumber,
Convert(NVarchar(50),asurv.CNumber)
as CNumber,
               asurv.AId,
               asurv.SIds,
               asurv.CSymb
as SCSymb,
               scs.CSymb
as SurCSymb
from AssignedValues asurv
left outer
join SurCSymbs scs
on    scs.PID
= asurv.PID
and scs.SIds
= asurv.SIds
and scs.CSymb
= asurv.CSymb
--Error is thrown at this statement
insert into @ExcludableTempValue
select *
from FullDetails;
with SurHavingSym as (   
select distinct est.PID,
                        est.ENumber,
                        est.CNumber,
                        est.AId
from @ExcludableTempValue est
where est.SurCSymb
is not
null
delete @ExcludableTempValue
from @ExcludableTempValue est
inner join SurHavingSym shs
on    shs.PID
= est.PID
and shs.ENumber
= est.ENumber
and shs.CNumber
= est.CNumber
and shs.AId
= est.AId;
insert @retExcludables(PID, ENumber, CNumber, AId)
select distinct est.PID,
Convert(nvarchar(50),est.ENumber)
ENumber,
Convert(nvarchar(50),est.CNumber)
CNumber,
                        est.AId      
from @ExcludableTempValue est 
RETURN
ROLLBACK
TRAN
I have tried by converting the columns and also validated the input data set for any white spaces or special characters.
For the same input data, it was working fine till yesterday but suddenly it started throwing the exception.

Answer 1

Remember, the CTE isn't executing the SQL exactly in the order you read it as a human (don't get too picky about that statement, it's at least partly true enough to say it's partly true), nor are the line numbers or error messages easy to read: a mismatch
in any of the joins along the way leading up to your insert could be the cause too.  I would suggest posting the table definition/DDL for:
- PSMetadata, in particular PSRowID, but just post it all
- tblProjectSur, in particularcolumns CSymbID and TblProjSurSurID
- cSymbLookup, in particular column CSymbID
- PSRow, in particular columns SampleRowID, PID,
- PSAuser and PSUserSur, in particualr all the USERID and RowID columns
- SurCSymbs, in particular colum SIDs
Also, a diagnostic query along these lines, repeat for each of your tables, each of the columns used in joins leading up to your insert:
Select count(asurv.sid) as count all
, count(case when asurv.sid between 0 and 9999999999 then 1 else null end) as ctIsaNumber
from SurvCsymb
The sporadic nature would imply that the optimizer usually chooses one path to the data, but sometimes others, and the fact that it occurs during the insert could be irrelevant, any of the preceding joins could be the cause, not the data targeted to be inserted.