Description of problem:
I ran sudo dnf upgrade --refresh in a Fedora 34 KDE Plasma installation on 2021-2-19. The update included systemd-247.3-3.fc34.x86_64. While dnf was upgrading systemd, an error message "systemd[1]: Caught <ABRT>, dumped core as pid 5419." was shown.
The journal at the time of that crash was the following.

Feb 19 09:13:10 audit[5419]: ANOM_ABEND auid=4294967295 uid=0 gid=0 ses=4294967295 subj=system_u:system_r:init_t:s0 pid=5419 comm="systemd" exe=2F7573722F6C69622F73797374656D642F73797374656D64202864656C6574656429 sig=6 res=1
Feb 19 09:13:10 systemd-coredump[5420]: Due to PID 1 having crashed coredump collection will now be turned off.
Feb 19 09:13:10 systemd-coredump[5420]: Process 5419 (systemd) of user 0 dumped core.
                                        
                                        Stack trace of thread 5419:
                                        #0  0x00007fe0b29e058b kill (libc.so.6 + 0x3d58b)
                                        #1  0x000055d90d62948e crash (/usr/lib/systemd/systemd (deleted) + 0x4848e)
                                        #2  0x00007fe0b2b86960 __restore_rt (libpthread.so.0 + 0x13960)
                                        #3  0x00007fe0b29e0292 __GI_raise (libc.so.6 + 0x3d292)
                                        #4  0x00007fe0b29c98a4 __GI_abort (libc.so.6 + 0x268a4)
                                        #5  0x00007fe0b2a22cd7 __libc_message (libc.so.6 + 0x7fcd7)
                                        #6  0x00007fe0b2a2a94c malloc_printerr (libc.so.6 + 0x8794c)
                                        #7  0x00007fe0b2a2b82c unlink_chunk (libc.so.6 + 0x8882c)
                                        #8  0x00007fe0b2a2bc21 _int_free (libc.so.6 + 0x88c21)
                                        #9  0x00007fe0b2a2fa48 __GI___libc_free (libc.so.6 + 0x8ca48)
                                        #10 0x00007fe0b2ec9ed7 n/a (/usr/lib/systemd/libsystemd-shared-247.so (deleted) + 0x1deed7)
Feb 19 09:13:10 abrt-dump-journal-core[865]: Failed to obtain all required information from journald
Feb 19 09:13:10 systemd[1]: Caught <ABRT>, dumped core as pid 5419.
Feb 19 09:13:10 systemd[1]: Freezing execution.
Feb 19 09:13:10 systemd-oomd[735]: Failed to connect to /run/systemd/io.system.ManagedOOM: Connection refused
Feb 19 09:13:10 systemd-oomd[735]: Failed to acquire varlink connection
Feb 19 09:13:10 systemd-oomd[735]: Event loop failed: Connection refused
Feb 19 09:13:10 plasmashell[1285]: org.kde.plasma.notifications: Notification to send to backend contains invalid XML:  "Opening and ending tag mismatch." line 1 col 294
Feb 19 09:13:12 plasmashell[1285]: file:///usr/share/plasma/plasmoids/org.kde.plasma.notifications/contents/ui/NotificationPopup.qml:116:15: QML QQuickItem: Binding loop detected for property "height"
Feb 19 09:13:12 dbus-broker-launch[803]: Noticed file-system modification, trigger reload.
Feb 19 09:13:12 dbus-broker-launch[803]: ERROR launcher_reload_config @ ../src/launch/launcher.c +1164: Connection refused
Feb 19 09:13:12 dbus-broker-launch[803]:       launcher_on_dirwatch @ ../src/launch/launcher.c +154
Feb 19 09:13:12 pipewire[1706]: alsa-pcm 0x55ac0c6d7398: snd_pcm_status error: Broken pipe
Feb 19 09:13:12 systemd-udevd[5659]: Using default interface naming scheme 'v247'.
Feb 19 09:13:12 systemd-udevd[5658]: Using default interface naming scheme 'v247'.
Feb 19 09:13:13 abrt-notification[5700]: Process 5419 (systemd) crashed in __libc_free()

While dnf was cleaning up packages, Plasma segmentation faulted. The error that systemd had aborted was shown in the VT.
Broadcast message from systemd-journald...
systemd[1]: Caught <ABRT>, dumped core as pid 5419.
Broadcast message from systemd-journald...
systemd[1]: Freezing execution.

After several minutes of the system remaining on that message, I used sysrq+alt+r,e,i,s,u,b to kill all processes, sync, remount read-only and reboot.

The trace of the systemd crash showed a "corrupted double-linked list" in malloc_printerr at malloc.c:5626 in glibc-2.32.9000-29.fc34.x86_64 in frame 6.

Core was generated by `/usr/lib/systemd/systemd --switched-root --system --deserialize 31'.
Program terminated with signal SIGABRT, Aborted.
#0  0x00007fe0b29e058b in kill () at ../sysdeps/unix/syscall-template.S:120
120     ../sysdeps/unix/syscall-template.S: No such file or directory.
[Current thread is 1 (LWP 5419)]
(gdb) bt
#0  0x00007fe0b29e058b in kill () at ../sysdeps/unix/syscall-template.S:120
#1  0x000055d90d62948e in crash (sig=6) at ../src/core/main.c:224
#2  <signal handler called>
#3  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:49
#4  0x00007fe0b29c98a4 in __GI_abort () at abort.c:79
#5  0x00007fe0b2a22cd7 in __libc_message (action=action@entry=do_abort, 
    fmt=fmt@entry=0x7fe0b2b33806 "%s\n") at ../sysdeps/posix/libc_fatal.c:155
#6  0x00007fe0b2a2a94c in malloc_printerr (
    str=str@entry=0x7fe0b2b31a15 "corrupted double-linked list") at malloc.c:5626
#7  0x00007fe0b2a2b82c in unlink_chunk (p=p@entry=0x55d90f2c5050, av=0x7fe0b2b66a00 <main_arena>)
    at malloc.c:1614
#8  0x00007fe0b2a2bc21 in _int_free (av=0x7fe0b2b66a00 <main_arena>, p=0x55d90f2c5050, 
    have_lock=<optimized out>) at malloc.c:4564
#9  0x00007fe0b2a2fa48 in __GI___libc_free (mem=<optimized out>) at malloc.c:3309
#10 0x00007fe0b2ec9ed7 in event_free (e=0x55d90f04b330) at ../src/libsystemd/sd-event/sd-event.c:292
#11 0x00007fe0b2eca1ab in event_free (e=<optimized out>) at ../src/libsystemd/sd-event/sd-event.c:258
#12 0x000055d90d671277 in manager_free (m=0x55d90f04b6b0) at ../src/core/manager.c:1406
#13 0x000055d90d626572 in manager_free (m=<optimized out>) at ../src/core/manager.c:1352
#14 main (argc=<optimized out>, argv=<optimized out>) at ../src/core/main.c:2878
 

Version-Release number of selected component:
systemd-247.3-2.fc34

Additional info:
reporter:       libreport-2.14.0
backtrace_rating: 4
cgroup:         0::/init.scope
cmdline:        /usr/lib/systemd/systemd --switched-root --system --deserialize 31
crash_function: event_free
executable:     /usr/lib/systemd/systemd
journald_cursor: s=c26fcd8a329a4568939359e3e9030ac0;i=212abf;b=fa88c0ce8a664451b1bcb879247150aa;m=83a0b96e;t=5bbb10d4fa511;x=cf8e1d03a2d8d996
kernel:         5.11.0-156.fc34.x86_64
rootdir:        /
runlevel:       N 5
type:           CCpp
uid:            0

Truncated backtrace:
Thread no. 1 (2 frames)
 #10 event_free at ../src/libsystemd/sd-event/sd-event.c:292
 #12 manager_free at ../src/core/manager.c:1406