Occasional error when running sp_BlitzFirst
You'll get that error when one of the columns in the select won't fit into the destination table.  

Tracking it down can take some time, but it's just like kindergaten - try and fit the shapes in the holes.

If you compare #Variable_info to #stored_proc_info, you will probably find that one of the columns in #variable_info is larger than its corresponding column in #stored_proc_info.

I can't find #variable_info in my version of the sp_blitzcache, so I can't tell which columns don't match.

Often, I track it down by running a query like this:

SELECT len(vi.SPID) as spid, len(vi.SqlHandle) as handle, len(vi.QueryHash as hash), len(vi.variable_name) as variable_name, 
len(vi.variable_datatype) as data_type, len(vi.compile_time_value) as compile_time, len(vi.proc_name) as proc_name
FROM #variable_info AS vi;


and looking for values that are too big.