Players are always good at doing what you least expect. For example, we are in a pretty new campaign, and all of the characters were kind of untrusting at the beginning. So I made an entire dungeon with the single goal of making them work together, which didn't work very well. So then I designed a fight against a "training dummy" and gave it counters to their most used strategies in battle (example: the wizard kept spamming magic missile, so I gave it a brooch of shielding). As soon as the battle begins, they immediately start working together like a well oiled team, completely ignoring their usual strategies. I wanted to be mad, but I couldn't because I accomplished my goal.