drupal is welcoming other user after login

Question

I have website built on drupal 6.26.
Now i am facing a trouble that sometimes when the user logs in, the website is showing him Welcome message to another user!! for example if you login using user X, after reload of the page, it will show you a message of "Welcome Y" and shows the profile page of Y. fortunately, it shows profile Y but without Editing permission. and if you try to visit X's profile, it will be editable!!.
Any idea what could cause such ambiguity??

Answer 1

Thanks a lot guys, I found the problem,,, the caching of the login block was set to 1, i just made it no-cache for this block

mysql> select * from blocks where module like '%ajax%';
+-----+---------------+-------+---------+--------+--------+-------------+--------+----------+------------+-------+--------+-------+
| bid | module        | delta | theme   | status | weight | region      | custom | throttle | visibility | pages | title  | cache |
+-----+---------------+-------+---------+--------+--------+-------------+--------+----------+------------+-------+--------+-------+
|  35 | ajax_register | 0     | sky     |      1 |    -40 | rightheader |      0 |        0 |          0 |       |  |     1 |
|  39 | ajax_register | 0     | garland |      0 |      0 |             |      0 |        0 |          0 |       |        |     1 |
+-----+---------------+-------+---------+--------+--------+-------------+--------+----------+------------+-------+--------+-------+
2 rows in set (0.00 sec)

mysql> update blocks set cache=-1 where bid=35;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> update blocks set cache=-1 where bid=39;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0