Classic story: Once, a long time ago, there was a PnP campaign where the protagonists were all Mages (or something along those lines). They got to high levels, pooled their resources, and built themselves a Mage Tower made out of diamond or adamantite, or similarly durable material. The DM decided that they'd had it too easy for too long (no really, ya think?), and decided to give them a challenge. In the dead of night, a small army of high-level Wizard Slayers laid siege to the tower, and erected a special Antimagic Shell around it, so the wizards couldn't teleport away. Then the Wizard Slayers started attacking the tower itself--because of the nature of its construction, it would most likely take days to effect a breach, but they
would get through eventually. And due to the extremely high Magic Resistance of the attackers, the mages inside would find it very difficult to harm them, or even scare them away, without being peppered with arrows for their trouble. So, what to do, what to do?
After some consultation amongst themselves, all the mages sat down and furiously started creating spell scrolls. Scroll after scroll went by, as they gave up sleep in order to write more spells. By the third day, they had a stack of over 100 scrolls copied. They memorized some Invisibility and Protection from Missiles spells, got some rest, and the next morning they were ready. They cast their buffs, appeared at the windows, and read off multiple scrolls of
Haste at their attackers. The Wizard Slayers paused momentarily, rather surprised at this turn of events, and then grinned and attacked all the faster. The Mages kept reading Hastes at them, and the army stopped shooting and let them do it, while continuing to batter down their tower. Only after the last scroll was gone did the players remind the DM that since Haste is a friendly spell, it can't be blocked by Magic Resistance . . . and each casting of it ages the recipient(s) by one year.
DM facepalm!
And then, the smiling Mages grabbed their staves and their daggers, trooped down the stairs & out the main gate, and proceeded to clobber the snot out of the geriatric Wizard Slayers in hand-to-hand combat.