I saw sometimes crashes while connecting to a series networks (using a script) upon mIRC start. They occurred shortly after a connection was refused (ON ERROR).

mIRC first triggers the ON ERROR and after that the ON DISCONNECT.
In the first I resetted the connection queue entries' 'status' flag from busy to queue which made the script not find any connection attempt in progress in the followed up on disconnect event and forced into thinking it was remotely disconnected so it did a reconnection attempt, which removes any existing jobs in the queue, also closing existing status windows, causing the crash.

It's easy reproducable on my machine:
Code:
on *:DISCONNECT:{ window -c "Status Window" }

mIRC 6.20 gives an invalid page fault on 0167:00420d89

Registers:
EAX=00000000 CS=0167 EIP=00420d89 EFLGS=00010246
EBX=00000064 SS=016f ESP=0087ee34 EBP=bff5582a
ECX=00000019 DS=016f ESI=00fe71a0 FS=494f
EDX=00000064 ES=016f EDI=00fe71a0 GS=0000
Bytes in CS:EIP:
f3 ab 8b cb 83 e1 03 f3 aa 8b 44 24 10 85 c0 5f
Stackdump:
00000000 00000000 00fe6078 004595f6 00fe6fac 005c6dec 005c6dec 00000000 00fe6078 00000003 0056908d 00fe6078 005a4bec 00000000 00000001 00458bf8