Weird bug in Plasma v Wayland: The icons-only task switcher will occasionally lock up. It'll cause anything on that same panel to lock up as well, but everything else should be fine.
Figured out that it seems to be specific to it trying (and probably failing) to generate a thumbnail for a window when you hover over an icon for a minimised application. Just turning off thumbnails works as a workaround. Apparently, this is fixed in KDE 6, so probably no real reason to report it.