I think the main errors happen with “voltooide deelwoorden” (past participle). Then you need mnemonic devices like “'t kofschip” to know whether it’s t or d (or determine it using what you would say in the past time of the verb). It doesn’t help that e.g., “gebeurt” and “gebeurd” both are correct depending on the tense used.
Also the fact that the t drops when the verb is inversed in the 2nd person singular present tense, and not e.g., past tense (“Je wordt” but “Word je”) is a weird rule.
It’s not thát complicated and if you pay attention, you should be able to get it all right. That’s why I think such mistakes are more a sign of carelessness and not of stupidity.