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)
As you will see below, most Internal Errors seem to be caused by WEAPONS and/or WARHEADS.
Weapons:
1. Typo for a weapon entry on a vehicle/infantry/aircraft/building so that it points to a non-existent weapon.
2. A weapon that is used in a game mode but not parsed by (attached to) another unit in the main rules file.
3. Cluster/shrapnel weapon not parsed by (attached to) another unit.
Warheads:
4. Missing warhead on a weapon.
5. A warhead with CellSpread bigger than 11.
6. Setting off a warhead with a CellSpread that is also Temporal=yes.
Other IE causes:
7. An MCV becomes neutral: Mind-control of an MCV is relinquished to a defeated player (RA2/YR only). The workaround is to make MCVs unable to be mind-controlled (this is done in the YR 1.002 UMP). Neutral Construction Yards do not seem to be a problem.
8. If a falling paratrooper (who has nearly reached the ground) is killed by an area-effect mutation weapon you will get an IE. Point based mutations seem to be okay and the GM superweapon seems incapable of killing falling paratroopers. If you have an area-effect mutation weapon then you should ensure that all paratroopers are immune to it.
9. HoverPad=yes. If the AI uses a Weather Storm or Nuclear Missile superweapon and a building with HoverPad=yes exists, the game will suffer an IE. Basically, don't use HoverPad=yes.
10. Omitting MakeInfantry=X on InfDeath 9's animation (InfantryMutate=GENDEATH) can sometimes cause an internal error. See the final note on MakeInfantry for more information.
11. Using the 'sell unit' super weapon on a Tank-Bunkered unit breaks the Tank Bunker. Attempting to sell or destroy the broken Tank Bunker causes an internal error. The moral of the story? Either don't include the 'sell unit' super weapon, or don't include the Tank Bunker.
12. Adding a new harvester and not making AI Trigger for it.
13. The UndeploysInto=/DeploysInto= tag(s) can be used on infantry to get two or three infantry units to be treated as the same type by the 'select all of type' button. However, if such an infantry unit is modified (in any way) by an mp game mode file (and probably by maps as well, although this is untested) then an IE will occur when a human player views an AI-owned War Factory.
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.