I recently upgraded to mysql 5.0.26 and have been regretting it every since. One of the things which relies on it because the perms are wrong so I issued the following statement:
mysql> grant update on somedb.sometable to someuser@localhost;
ERROR 2013 (HY000): Lost connection to MySQL server during query
This happens every single time. Mysql silently dies. The mysql client returns that the connection has been lost and I have to restart the database.
The mysqld.err log shows the following:
061208 10:48:00  InnoDB: Started; log sequence number 0 5711233
061208 10:48:00 [Note] /usr/sbin/mysqld: ready for connections.
Version: '5.0.26'  socket: '/var/run/mysqld/mysqld.sock'  port: 3306  Gentoo Linux mysql-5.0.26-r1
mysqld got signal 11;
This could be because you hit a bug. It is also possible that this binary
or one of the libraries it was linked against is corrupt, improperly built,
or misconfigured. This error can also be caused by malfunctioning hardware.
We will try our best to scrape up some info that will hopefully help diagnose
the problem, but since we have already crashed, something is definitely wrong
and this may fail.

It is possible that mysqld could use up to 
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_connections = 92783 K
bytes of memory
Hope that's ok; if not, decrease some variables in the equation.

Attempting backtrace. You can use the following information to find out
where mysqld died. If you see no messages after this, something went
terribly wrong...
Cannot determine thread, fp=0xac62bc18, backtrace may not be correct.
Stack range sanity check OK, backtrace follows:
New value of fp=(nil) failed sanity check, terminating stack trace!
Please read http://dev.mysql.com/doc/mysql/en/Using_stack_trace.html and follow instructions on how to resolve the stack trace. Resolved
stack trace is much more helpful in diagnosing the problem, so please do 
resolve it
Trying to get some variables.
Some pointers may be invalid and cause the dump to abort...
thd->query at 0x8b6f590 = grant update on somedb.sometable to someuser@localhost
The manual page at http://www.mysql.com/doc/en/Crashing.html contains
information that should help you find out what is causing the crash.
061208 10:48:24  InnoDB: Started; log sequence number 0 5711233
061208 10:48:24 [Note] /usr/sbin/mysqld: ready for connections.
Version: '5.0.26'  socket: '/var/run/mysqld/mysqld.sock'  port: 3306  Gentoo Linux mysql-5.0.26-r1
I'm not sure if this is a bug or what but I am really getting tired of things breaking when I upgrade them. I'm supposed to be running gentoo "stable".

I'm also not sure what I can do about this. I have to run that grant statement or the webapp won't work. I don't understand why mysql is crashing. The table does exist (and even if it didn't the whole mysqld shouldn't crash!).

Any ideas?