Hello Patrick.

Right now I found a way to reproduce this bug.

You need two users with 3 grands.

test1 to database test1
test2 to databases test2 and mysqlslap

my.cnf settings:
[mysqld]
max_connections=50
max_user_connections=25
log-warnings=10


Changes to mysql.user table:

set max_user_connections for user test2 to 100.

Now you have to start 3 processes:

# it is help to have Too many connections error (max_connections field)
mysqlslap  --concurrency=10 --iterations=1000  --auto-generate-sql --engine=innodb  --number-of-queries=1000 --user=test1 -ptest1pass --create-schema=test1

# it is help to have 1226 User 'test2' has exceeded the 'max_user_connections' resource (max_user_connections field from mysql.user)
mysqlslap --concurrency=5 --iterations=1000 --auto-generate-sql --engine=innodb --number-of-queries=1000 --user=test2 -ptest2


# this one overloading all limits (max_connections and max_user_connections)
mysqlslap  --concurrency=100 --iterations=100 --number-int-cols=2 --number-char-cols=3 --auto-generate-sql  --engine=blackhole,myisam,innodb  --auto-generate-sql-add-autoincrement --auto-generate-sql-load-type=mixed  --number-of-queries=100 --user=test2 -ptest2pass  --create-schema=test2

It takes time, while bug will show itself.

You can speed it up by:
mysql> set global max_connections =20;

And right away you will see:
mysqlslap: Error when connecting to server: 1226 User 'test2' has exceeded the 'max_user_connections' resource (current value: 100)
mysqlslap: Error when connecting to server: 1226 User 'test2' has exceeded the 'max_user_connections' resource (current value: 100)
mysqlslap: Error when connecting to server: 1040 Too many connections
mysqlslap: Error when connecting to server: 1226 User 'test2' has exceeded the 'max_user_connections' resource (current value: 100)
mysqlslap: Error when connecting to server: 1226 User 'test2' has exceeded the 'max_user_connections' resource (current value: 100)
mysqlslap: Error when connecting to server: 1226 User 'test2' has exceeded the 'max_user_connections' resource (current value: 100)
mysqlslap: Error when connecting to server: 1040 Too many connections
mysqlslap: Error when connecting to server: 1226 User 'test2' has exceeded the 'max_user_connections' resource (current value: 100)
mysqlslap: Error when connecting to server: 1226 User 'test2' has exceeded the 'max_user_connections' resource (current value: 100)
mysqlslap: Error when connecting to server: 1226 User 'test2' has exceeded the 'max_user_connections' resource (current value: 100)
mysqlslap: Error when connecting to server: 1226 User 'test2' has exceeded the 'max_user_connections' resource (current value: 100)
mysqlslap: Error when connecting to server: 1040 Too many connections
mysqlslap: Error when connecting to server: 1226 User 'test2' has exceeded the 'max_user_connections' resource (current value: 100)
mysqlslap: Error when connecting to server: 1226 User 'test2' has exceeded the 'max_user_connections' resource (current value: 100)


Thats it.
Now you can interrupt all mysqlslaps and start new one: (don't restart mysql)

[root@nestor ~]# mysqlslap --concurrency=10 --iterations=1000 --auto-generate-sql --engine=innodb --number-of-queries=1000 --user=test2 -ptest2test 
mysqlslap: Error when connecting to server: 1226 User 'test2' has exceeded the 'max_user_connections' resource (current value: 100)
mysqlslap: Error when connecting to server: 1226 User 'test2' has exceeded the 'max_user_connections' resource (current value: 100)
mysqlslap: Error when connecting to server: 1226 User 'test2' has exceeded the 'max_user_connections' resource (current value: 100)
mysqlslap: Error when connecting to server: 1226 User 'test2' has exceeded the 'max_user_connections' resource (current value: 100)


[root@nestor ~]# mysql --user=test2 -ptest2test -e 'show processlist'
ERROR 1226 (42000): User 'test2' has exceeded the 'max_user_connections' resource (current value: 100)

and process list by root:
[root@nestor ~]# mysqladmin pr
+-------+-------+-----------+-----------+---------+------+--------------+------------------------------------------------------------------------------------------------------+-----------+---------------+-----------+
| Id    | User  | Host      | db        | Command | Time | State        | Info                                                                                                 | Rows_sent | Rows_examined | Rows_read |
+-------+-------+-----------+-----------+---------+------+--------------+------------------------------------------------------------------------------------------------------+-----------+---------------+-----------+
| 14395 | test2 | localhost | mysqlslap | Sleep   | 14   |              |                                                                                                      | 0         | 0             | 2         |
| 14568 | test2 | localhost | mysqlslap | Query   | 1    | Sending data | SELECT intcol1,charcol1 FROM t1                                                                      | 242       | 0             | 243       |
| 14569 | test2 | localhost | mysqlslap | Query   | 1    | Sending data | SELECT intcol1,charcol1 FROM t1                                                                      | 242       | 0             | 243       |
| 14570 | test2 | localhost | mysqlslap | Query   | 1    | Sending data | SELECT intcol1,charcol1 FROM t1                                                                      | 336       | 0             | 337       |
| 14571 | test2 | localhost | mysqlslap | Query   | 0    | update       | INSERT INTO t1 VALUES (866596855,'naQuzhMt1IrZIJMkbLAKBNNKKK2sCknzI5uHeGAgQuDd5SLgpN0smODyc7qorTo1Qa | 0         | 0             | 1         |
| 14572 | test2 | localhost | mysqlslap | Query   | 1    | Sending data | SELECT intcol1,charcol1 FROM t1                                                                      | 251       | 0             | 252       |
| 14573 | test2 | localhost | mysqlslap | Query   | 1    | Sending data | SELECT intcol1,charcol1 FROM t1                                                                      | 288       | 0             | 289       |
| 14615 | root  | localhost |           | Query   | 0    |              | show processlist                                                                                     | 0         | 0             | 1         |
+-------+-------+-----------+-----------+---------+------+--------------+------------------------------------------------------------------------------------------------------+-----------+---------------+-----------+

How you can see - it generate 1226 error. There is no '1040 Too many connections' errors anymore.
And only ~10 active connections (max_user_connection set up to 100 by mysql.user)

I am still looking for source of this issue.
I will keep you up2date.

Regards,
Gor Martsen