He're a message I get when starting plasma-desktop (this piece is actually repeated several times; maybe, the pieces are not identical..):

--- 8< ---
ProcessControl: Application 'akonadiserver' returned with exit code 255 (Unknown error)
search paths:  ("/usr/local/bin", "/usr/bin", "/bin", "/usr/local/sbin", "/usr/sbin", "/sbin", "/usr/bin/core_perl", "/home/pilat/.gem/ruby/1.9.1", "/usr/sbin", "/usr/local/sbin", "/usr/local/libexec", "/usr/libexec", "/opt/mysql/libexec", "/opt/local/lib/mysql5/bin", "/opt/mysql/sbin") 
QSqlDatabasePrivate::removeDatabase: connection 'initConnection' is still in use, all queries will cease to work.
Database error: Cannot open database.
Last driver error: "QMYSQL: Unable to connect"
Last database error: "Can't connect to local MySQL server through socket '/home/pilat/.local/share/akonadi/socket-pilat-book/mysql.socket' (2)"
QSqlQuery::exec: database not open
Unable to open database "Can't connect to local MySQL server through socket '/home/pilat/.local/share/akonadi/socket-pilat-book/mysql.socket' (2) QMYSQL: Unable to connect"
"[
0: akonadiserver(_Z11akBacktracev+0x34) [0x4540e4]
1: akonadiserver() [0x454421]
2: /usr/lib/libc.so.6(+0x35340) [0x7f27c422e340]
3: /usr/lib/libc.so.6(gsignal+0x35) [0x7f27c422e2c5]
4: /usr/lib/libc.so.6(abort+0x148) [0x7f27c422f748]
5: /usr/lib/libQtCore.so.4(_Z17qt_message_output9QtMsgTypePKc+0x74) [0x7f27c5cff414]
6: akonadiserver(_ZN15FileDebugStream9writeDataEPKcx+0x9b) [0x45619b]
7: /usr/lib/libQtCore.so.4(_ZN9QIODevice5writeEPKcx+0xb4) [0x7f27c5d99b54]
8: /usr/lib/libQtCore.so.4(+0x1167ff) [0x7f27c5da47ff]
9: /usr/lib/libQtCore.so.4(_ZN11QTextStreamD1Ev+0x3b) [0x7f27c5dacdfb]
10: akonadiserver(_ZN7Akonadi13AkonadiServerC1EP7QObject+0x512) [0x4581f2]
11: akonadiserver(_ZN7Akonadi13AkonadiServer8instanceEv+0x47) [0x459477]
12: akonadiserver(main+0x1a0) [0x44d670]
13: /usr/lib/libc.so.6(__libc_start_main+0xf5) [0x7f27c421aa15]
14: akonadiserver() [0x44de71]
]
--- >8 ---

I have two questions here:

1. Why it even tries to use akonadiserver? I thought that disabling something via systemsettings would "inform" all sub-systems that the component is not available;

2. Why "unknown error"? Ok, not every plasmoid may read the system settings, but `akonadiserver` iself should "know" somehow that it's disabled by user and return appropriate status?

Reproducible: Always

Steps to Reproduce:
1. Disable everything in System Settings > Desktop Search
2. Put "StartServer=false" in ~/.config/akonadi/akonadiserverrc
3. Run plasma-desktop from Konsole.

Note: if it doesn't appear, try installing "Lock keys state (QML)" plasmoid (this is the only one I have added tot he panel beside default ones).