$cb() appearing to null the usage of data past the size of 4150 bytes. At first I was going to say that the number of lines of text (lines being defined as \r\n delimited lines) was limited, but I searched a little further and found out that it was actually the bytesize of the content of the clipboard that caused an error. Highlight and copy the following test data..
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This consists of 29 rows of 140 periods, 5 rows of 2 periods, and 1 row of 1 period, (feel free to count them, haha), which totals up to 35 lines consisting of 4071 bytes (\r\n included is 4139). To test this weakness, let us make use of mIRC's /clipboard -an command to add on periods as well as increase line count.
//clipboard -an . | echo -a $cb(0).len
As you execute the code above, you will notice the pattern of the size of the clipboard increasing by 3 (as expected) due to appending a period followed by \r\n; we see this pattern cut off after a size of 4150, something that does not occur when using v6.35. If you were to paste the contents of the clipboard into a word processor, you will see that we indeed did append those extra periods, however, mIRC totally disgarded them and rendered their usage/existance useless. This cutoff of 4150 seems to be consistent across different test strings.