Using VPPAdminTools, I spawned in numerous classes of infected to test the hat clipping I've noticed on official servers. This effect really breaks immersion when playing the game. For this report, I'll highlight one prime example, the zmbm_jacket class which comes in many color combinations. I found that every hat preset for city and for tier 1 hats clips with this infected model's head. Images attached with the relevant subclasses from cfgspawnabletypes and cfgpresets. I did find this zmbm_jacket class does not clip with the great helm, construction hat, or with the dirt bike helmets. Those are the only ones though. Solution may be to simply remove the hat preset from this zmbm_jacket model. Or, add orange vest with construction hat for plain clothes industrial look... or pair chest plate (vest slot) with great helm at castle locations.

If you're interested in more details for the other infected classes I can prepare a spreadsheet reviewing all of them. There are a few that also seem to clip with most hat options, while some are 50/50 with a few working hats and a few that clip.