Printing record types using format now has the side effect that future printing will signal errors.  This bug makes debugging other bugs hard because it's impossible to move up the stack.  Here's a transcript of the simplest reproducible test case I could find:

+VERBATIM+
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  Release 13.0.50 || SF || LIAR/x86-64

1 ]=> (load-option 'format)
;Loading "format.com"... done
;Value: format

1 ]=> (load-option 'sos)
;Loading "load.scm"...
;  Loading "sos-unx.pkd"... done
;  Loading "geneqht.com"... done
;  Loading "generic.com"... done
;  Loading "genmult.com"... done
;  Loading "tvector.com"... done
;  Loading "recslot.com"... done
;  Loading "slot.com"... done
;  Loading "class.com"... done
;  Loading "instance.com"... done
;  Loading "method.com"... done
;  Loading "printer.com"... done
;  Loading "macros.com"... done
;... done
;Value: sos

1 ]=> (format #true "~A" <bundle>)
#[record-type 12 <bundle>]
;Unspecified return value

1 ]=> <generic-procedure>
;Value:
;The object
;To continue, call RESTART with an option number:
; (RESTART 1) => Return to read-eval-print level 1.

2 error>
-VERBATIM-

Note that it's not necessary to print <bundle> or <generic-procedure> in particular.  Passing an instance of any record type to format will cause the bug in future printing.

I've only been able to reproduce this bug with SOS loaded.  The XML parsing library uses SOS, so this makes using that problematic.

The bug appeared (thanks, git bisect) somewhere between 3b536b9f5 and 8d244e6c0.  I can't be sure exactly which commit introduced the bug because all the builds in that range were broken before 8d244e6c0, at least in my testing.  I'm guessing that they were all part of one effort.

8d244e6c0 * Tweaks. [This one  builds.]
aa2f322b0 * Clear handler cache even on overwrites.
10cec1ac2 * Change calling convention for %make-tag.
122b8419f * Change UCD character sets to use their built-in predicates.
22e3ed00d * Simplify %record->leaf-type.
8b52e11ea * Change types to have subset links, and remember simple types.
61e11c4af * Revamp the representation of types.
3b536b9f5 * Initial draft of type-dispatcher.