Thanks for your bug report. I have not been able to reproduce an issue with +H so far, in v7.57, the latest beta, or previous versions of mIRC.

On which IRC network does this happen? I connected to, which is running InspIRCd 3.0, to test this.

Are you using v7.57 yourself? Your mIRC should be receiving the same mode changes as other clients. And if you part and rejoin the channel, you will receive the same channel history messages as well.

If you run another copy of mIRC v7.57 and join the same channel, so that you have two copies of mIRC on that channel, and then change the /mode +H setting, are you able to reproduce this issue? ie. make one of your own copies of mIRC crash?