MIL and her wizened cronies are very particular about cards and all the commensurate protocol.
We lived next door to one of MIL's old friends for a while (we live 200 miles away from MIL, it was a horrid coincidence!) and when ds1 was about 2mo she brought round a packet of little sleepsuits for him. They were lovely, I thanked her effusively and made sure that he was wearing one next time we saw her, at which point I thanked her again.
Two weeks later I received a snotty letter from MIL saying that our neighbour had phoned her to tell her how offended she was that we hadn't bothered to send her a "thank you card"!!!
Mind-boggling