Power Query occassionally connects, seems to load, but returns only 200 rows with 1 error

Question

I have a Power Query that i have used for few weeks, successfully, but occasionally requires being run a few times to return records.  Just lately it seems to require run more than a few times to make it work. 
When it doesn't return records , it seems to connects and start retrieving data, but returns only randomly like 100 - 200 rows with 1 error.  The error doesn't have any message. So its hard to troubleshoot from that perspective.
Query appends two other large and merges in a few others. The data sources are either csv files, Sharepoint Lists and tables in workbook.
I haven't identified anything in terms of environment or query changes that might cause this.
When it finally does retrieve all records,  it might be after i have done the following things, but i can't say for certain exactly what did the trick:
* to run query multiple times
*maybe simply changing anything in query resolves issue
* making upstream queries (that are merged or appended into this query)  to Not Load to worksheet or model, eg just have connection
I'd think this type of error and situation while not common is encountered by others.
Does this ring a bell for anyone, and if so, can you please shed some light on the reasons and resolutions for this?
Thanks!

Answer 1

If you click on "1 error" it should show the editor with a query that will select the rows with errors. This unfortunately might not show all errors for various reasons, but you should try it.
If that doesn't work, here's an example of how to retrieve error details.
Suppose the following query:
= Table.FromRecords({[A="B", B="C" + 1]})
Notice how we're using the + operator with a string and number. This will result in an error.
I can create a custom column that uses the 'try' operator over the B column. This returns a record with details which I then expand to retrieve the message.
let
Source = Table.FromRecords({[A="B", B="C" + 1]}),
#"Added Custom" = Table.AddColumn(Source, "Custom", each try [B]),
#"Expand Custom" = Table.ExpandRecordColumn(#"Added Custom", "Custom", {"HasError", "Error"}, {"Custom.HasError", "Custom.Error"}),
#"Expand Custom.Error" = Table.ExpandRecordColumn(#"Expand Custom", "Custom.Error", {"Message"}, {"Custom.Error.Message"})
in
#"Expand Custom.Error"
Let me know how this works for you.
Tristan