Add a known issue for TiFlash mpp.

tiflash/use-tiflash-mpp-mode.md

@@ -175,3 +175,18 @@ mysql> explain SELECT count(*) FROM test.employees;
 +------------------------------+----------+--------------+-----------------+---------------------------------------------------------+
 5 rows in set (0,00 sec)
 ```
+
+## Known issues of MPP
+
+In the current version, TiFlash uses the `start_ts` of a query as the unique key of the query. In most cases, the `start_ts` of each query can uniquely identify a query, but in the following cases, different queries have the same `start_ts`:
+
+- All queries in the same transaction have the same `start_ts`.
+- When you use [`tidb_snapshot`](/system-variables.md#tidb_snapshot) to read data at a specific historical time point, the same time point is manually specified.
+- When [Stale Read](/stale-read.md) is enabled, the same time point is manually specified.
+
+When `start_ts` cannot uniquely represent the MPP query, if TiFlash detects that different queries have the same `start_ts` at a given time, TiFlash might report an error. Typical error cases are as follows:
+
+- When multiple queries with the same `start_ts` are sent to TiFlash at the same time, you might encounter the `task has been registered` error. 
+- When multiple simple queries with `LIMIT` are executed continuously in the same transaction, once the `LIMIT` condition is met, TiDB sends a cancel request to TiFlash to cancel the query. This request also uses `start_ts` to identify the query to be canceled. If there are other queries with the same `start_ts` in TiFlash, these queries might be canceled by mistake. An example of this issue can be found in [#43426](https://github.com/pingcap/tidb/issues/43426).
+
+This issue is fixed in TiDB v6.6.0. It is recommended to use the [latest LTS version](https://docs.pingcap.com/tidb/stable).