Internal Error
A general error returned by the TS engine and it's derivates. The message itself doesn't say much, and can be caused by almost anything you do wrong, so it's impossible to say where exactly it came from when it hits your screen. If you get an internal error, all you can do is try to track down what changes you have made to cause it. Review your latest rules-changes and play again to see when it occurs. Make sure you always know what you have changed, the more things you change simultaneously, the harder it is to see what caused the IE. Also, just because you found a mistake and fixed it, that doesn't mean there can't be a second one that's still causing the error.
List of known reasons for Internal Errors (please expand)
Internal Errors caused by weapons:
1. Missing warhead on a weapon.
2. Typo for a weapon entry on a unit/infantry/aircraft/building so that it points to a non-existent weapon.
3. Cluster weapon not attached to another unit.
4. A warhead with CellSpread bigger than 10.
5. A weapon that is used in a game mode but not attached to another unit in the main rules file.
6. Setting off an area-effect warhead that is also Temporal=yes.
7. A vehicle that can deploy into a building becomes neutral from a player side (RA2/YR only) such as relinquishing mind-control of an MCV to a defeated player. The workaround is to make such vehicles unable to be mind-controlled.
See also: Reconnection Error
Software used to find Internal Errors
INI Checker is a very useful program that can check your rules, art and sound ini files for typos and missing references.