I've just remembered the batshit crazy mum from nursery, unfortunately for me, DS totally latched on to her little boy as his bestest ever friend in the WHOLE world, they were totally obsessed with each other.
"How lovely" I thought at first, and happily accepted batshit crazy mum's invitation to take the boys to the park after nursery.
The first 
was when we were sitting down in this park, the children went off playing, and she asked what kind of music I liked. Oh, any, I said, so she started playing some music on her phone, just chart/pop kind of stuff. And then the uncensored version of "Forget You" by Cee-lo came on, I hadn't heard it before, but basically instead of every time he says "forget you and forget her too" in the song the "forget" is replaced with "fuck". But instead of being even slightly embarrassed she just left it playing, fucking away in the middle of a children's park 
Then she kept offering me knock-off cigarettes which I didn't want, and then there was an awkward other park visit where her son started crying after an argument with another child and she kept alternately telling him to man up and stop crying and then to tell her why he was crying, or she'd slap him 
or throw him in the river
Anyway I decided maybe she was having a bad day and gave it one last shot where we accepted a playdate invitation. While the children were playing in the garden, her partner started rolling a joint and then just smoked it right in front of them Being terribly English I couldn't say anything at all and couldn't bring myself to leave because she had dinner cooking for the DC and I didn't want to appear rude but while they were eating she proceeded to roll another one and offer me some and then smoke it while complaining to me about everything that was wrong with her life apart from her amazing boyfriend. I was and just tried not to breathe and even moved pointedly to another chair but she carried on oblivious. After that, luckily, there was a good opportunity to leave and then I'm ashamed to say I just ignored all of her calls and texts and facebook messages and then her DS moved schools and I just never contacted them ever again 
DS has forgotten totally about the other boy now, thank god.
I remember she told me that her DS always latched on to another boy as a best friend though and then when he moved on from the group/nursery/etc they never stayed in touch, I was a bit :( for him because it was probably her they were staying away from.