Out-of-memory errors and similar errors are due to the server running out of memory (RAM). The cause is a bit more difficult to determine.
Note: Every Minecraft package has a set amount of RAM.
Using plugins will consume this RAM, and each user on the server will take up a chunk of it as well. Thus, the more plugins the server has, the fewer players can be online at the same time. In this case, lower the server's max player slots.
Using multi-world plugins is notorious for causing out-of-memory crashes because each added world consumes a lot of memory, even if no players are on it.
Other causes of out-of-memory errors include outdated or incompatible plugins with the server's version. It is always best to ensure that everything on the server is up to date to reduce the risk of this occurring.
Note: A 100% stable server is nearly impossible to have due to the nature of Minecraft/CraftBukkit servers and plugins.
It can also be caused by things in the world, such as mobs, lava, water, redstone, pistons, and entities, as these can cause memory leaks and need to be removed manually.