Inadherence to Standards
See the Wiki page on Token Specific Recommendation for the complete checklist on ERC20 weirdness.
In terms of smart contract development, it's important to follow standards. Standards are set to prevent vulnerabilities, and ignoring them can lead to unexpected effects.
Take for example binance's original BNB token. It was marketed as an ERC20 token, but it was later pointed out that it wasn't actually ERC20 compliant for a few reasons:
It prevented sending to 0x0
It blocked transfers of 0 value
It didn't return true or false for success or fail
The main cause for concern with this improper implementation is that if it is used with a smart contract that expects an ERC-20 token, it will behave in unexpected ways. It could even get locked in the contract forever.
Although standards aren't always perfect, and may someday become antiquated, they foster the most secure smart contracts.
Sources
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