Label with text exceeding certain limit draws text several times with different horizontal positions, resulting in garbage:
label .lab -text [string repeat "Abc" 16000]
On my system, repeat count 6000 is OK and 7000 and higher draws garbage.
Tested with 8.5.1 (latest tclkit I found for 32bit Linux) and 8.4.12.
Looking through existing bug reports, the closest match I found was [ 547593 ] Error creating Label > 32k pixels , which was closed years ago.