When a file contains lines bigger than 999 characters and /fseek -r <name> <pattern> is used, mirc crashes. Here's a command to reproduce the bug:
//.remove test.test | bset -t &a 1 $str(a,500) | bcopy &a $calc($bvar(&a,0) + 1) &a 1 -1 | bwrite test.test -1 -1 &a | fopen a test.test | fseek -r a /b/ | fclose a
The green part writes a 1000-character line to the file. The red part is the cause of the crash. Changing "500" to a smaller number doesn't crash mirc.