Version:           2.5.5 (using KDE KDE 3.5.5)
Installed from:    Debian testing/unstable Packages
OS:                Linux

Under quite specific circumstances, trying to replace auto-indentation in kate can cause a crash.

You need to be in C-Style autoindentation, and tabs not spaces. Although if you swap spaces and tabs around, the bug might still exist. Here is a test case I created:

{
\tLine indented by one tab;
\tTab + no semicolon
\tPut four spaces [    ] into the clipboard.
\tPress enter at the end of this line, backspace, Ctrl-V, and then Ctrl-Z.

\tLook! The cursor is gone! Type something for explosion!
}

Where \t represents a tab.

To reproduce:

Load config as above, and load the test case.
Follow the instructions in the test case.
Kate will crash, with the following backtrace:

#0  0xb6f5542a in QString::insert () from /usr/lib/libqt-mt.so.3
#1  0xb6371fd2 in KateTextLine::insertText () from /usr/lib/kde3/libkatepart.so
#2  0xb63fbd22 in KateDocument::editInsertText () from /usr/lib/kde3/libkatepart.so
#3  0xb64081ff in KateDocument::insertText () from /usr/lib/kde3/libkatepart.so
#4  0xb6408778 in KateDocument::insertText () from /usr/lib/kde3/libkatepart.so
#5  0xb64188c5 in KateDocument::typeChars () from /usr/lib/kde3/libkatepart.so
#6  0xb6432893 in KateViewInternal::keyPressEvent () from /usr/lib/kde3/libkatepart.so
#7  0xb6411668 in KateViewInternal::eventFilter () from /usr/lib/kde3/libkatepart.so
#8  0xb6c38302 in QObject::activate_filters () from /usr/lib/libqt-mt.so.3
#9  0xb6c38380 in QObject::event () from /usr/lib/libqt-mt.so.3
#10 0xb6c6f612 in QWidget::event () from /usr/lib/libqt-mt.so.3
#11 0xb6bd0c26 in QApplication::internalNotify () from /usr/lib/libqt-mt.so.3
#12 0xb6bd2bda in QApplication::notify () from /usr/lib/libqt-mt.so.3
#13 0xb7381e0e in KApplication::notify () from /usr/lib/libkdecore.so.4
#14 0xb6b64495 in QApplication::sendSpontaneousEvent () from /usr/lib/libqt-mt.so.3
#15 0xb6b556f9 in QETWidget::translateKeyEvent () from /usr/lib/libqt-mt.so.3
#16 0xb6b61723 in QApplication::x11ProcessEvent () from /usr/lib/libqt-mt.so.3
#17 0xb6b77d02 in QEventLoop::processEvents () from /usr/lib/libqt-mt.so.3
#18 0xb6beb179 in QEventLoop::enterLoop () from /usr/lib/libqt-mt.so.3
#19 0xb6beaf9a in QEventLoop::exec () from /usr/lib/libqt-mt.so.3
#20 0xb6bd27bf in QApplication::exec () from /usr/lib/libqt-mt.so.3
#21 0xb7ed1c96 in kdemain () from /usr/lib/libkdeinit_kate.so
#22 0x08048482 in ?? ()
#23 0x00000001 in ?? ()
#24 0xbf8dc154 in ?? ()
#25 0xbf8dc0d8 in ?? ()
#26 0x080484f9 in ?? ()
#27 0xb7c91c8c in ?? () from /lib/tls/libc.so.6
#28 0xbf8dc0e0 in ?? ()
#29 0xbf8dc128 in ?? ()
#30 0xb7c9bea8 in __libc_start_main () from /lib/tls/libc.so.6
#31 0xb7c9bea8 in __libc_start_main () from /lib/tls/libc.so.6
#32 0x080483d1 in ?? ()

This seems to happen whenever the Autoindenter is about its business - when you press return, it inserts two extra tabs because there's no semicolon. When you backspace and paste in four spaces, it replaces those spaces - also in an odd way (you probably want to check that out ;) ) and when you undo, it goes to an intermediate step - after pasting, but before replacing.
At this point, the cursor disappears, and trying to type anything will produce the segfault. Note that pressing the cursor keys doesn't cause a crash.

Not a particularly common bug, I expect, but it does crop up when trying to do odd things with the autoindenter. (Trying to odd things being, using tabs for indentation and spaces for formatting)