Anomalies and Game Limits
This is a page for oddities, arbitrary limits, and other potentially-unintuitive behaviour that is nonetheless clearly working as intended and not bugged.
This page is under construction.
Great Circle Bug
The shortest distance between two points on the surface of a sphere is called a Great Circle. The wayfinding algorithm used by XCom aircraft has unfortunately never heard of this. XCom aircraft tend to move toward, and then along, lines of latitude. This is inefficient in terms of time, fuel, range and time on station. The inefficiency is most evident when crossing over the poles, and least evident when moving east-west or west-east.
As a workaround, you can set manual waypoints to approximate the Great Circle route.
Note that this does not seem to affect combat interception of moving UFOs, or at least not as much. Intercept courses seem to be continually updated, and head straight toward the current position of the UFO (also not an optimum algorithm, but that's another story).
Side-On Intercept Bug
Unlike fighter controllers since at least the 1930s, X-Com ground controllers and/or onboard avionics are incapable of calculating a predictive course for a target that is moving, at a known course and speed, laterally relative to its bearing as seen from the intercept base (so that the bearing from the base is constantly changing). In other words, in any situation except when the alien craft is heading directly toward/away from the X-Com craft/base, a non-optimum interception course is used. Instead, X-Com craft simplistically just keep heading toward the current bearing toward the target. This results in inefficient parabolic intercept courses, rather than efficient straight line courses toward a predicted intercept point. Parabolic courses often fail to intercept, or give very brief interception windows, that would have been avoided by a proper predictive, straight-line course.
Head-On Intercept Bug
It is not possible for a slower-moving X-Com craft to intercept a faster-moving Alien craft, even if the X-Com craft is directly ahead of the alien craft and waiting in position to intercept it. This is true whether the X-Com craft is stationary, or flying in the same heading as the alien craft and at nearly the same speed. Even if the alien craft moves right through the X-Com craft's exact position while travelling only slightly faster than the X-Com craft. Interception only occurs if the alien craft voluntary slows down for long enough for the X-Com craft to catch it.
(It's true that such interceptions would be difficult, whether attempted head-on at high closing speed - extremely brief period to fire - or tail-on at low closing speed - very easy for the alien to avoid entering X-Com craft's weapon arcs by breaking away. So it could be argued that making these interceptions impossible is just a simplification of the fact that they would be much more difficult than normal interception combat.)
Instant Getaway Bug
If you park an interception craft directly over a touched-down alien craft, the alien craft will still escape, if the alien craft has superior maximum speed. The alien craft basically seems to instantly accelerate to its maximum speed, with no concept of progressive acceleration. Interception only occurs if the alien craft voluntary slows down for long enough for the X-Com craft to catch it. This probably has the same root cause as the Head-On Intercept Bug (arguably it is a special case of the Head-On Intercept Bug).
When loading up your Avenger for a massive UFO assault or arming soldiers for a Base Defence from your overflowing stores, you will likely hit this limit.
You only get a max of 80 items, and you don't get to choose which ones, so you may end up with 80 clips and no rifles for the base defence.
Another bug concerning this limit happens when there are unresearched (alien) items in your stores during a base defence mission. Even though the item cannot be used during the mission, the game still allows you to equip your soldiers with them. This rarely happens as the alien items are very low on the item list, but if your base is lean and mean with under 80 items total, everything will spawn regardless of research.
The purchase limit for any item you buy via the purchase screen is 255 per line item, because it is a one-byte field. However, the impact of this bug is small, for if you have space remaining in your 100-item transfer queue, you can immediately order another 255 of said item as another line item.
Soldier Recruiting Limit
There is a hard limit in the game of 250 X-COM soldiers in total across all bases (not 250 per base).
The error message that pops up when you try to hire more is:
"NO MORE SOLDIERS ALLOWED"
"You have already recruited the maximum number of soldiers."
Soldier Battlescape Limit
In the Battlescape, there is a limit of 40 X-COM soldiers that can participate in any battle. Tanks/HWPs each count as 4 soldiers against this limit. Unless you use custom-modified aircraft, you will only see this limit in a Base Defence mission. In a Base Defence mission, all tanks/HWPs will be deployed first, in preference to soldiers.
Manufacturing Completion Time Display Bug
If you start a new manufacturing project at 00:01, and the screen displays a completion time of 5 hours, you might expect the work to complete at 05:00 or 05:01. In fact it will complete at 06:00, one hour longer than indicated.
The reason for this bug is that the time left to complete a project is decremented by the game by one hour, every hour, on the hour(XX:00).
This display behaviour is consistent with what is shown while manufacturing is in progress. For example, if time remaining is shown as 0 hours, the work will complete at the beginning of the next hour. In effect, the displayed number is the integer-truncated form of the real time remaining. In the first example given above, the real time to completion is 5 hrs 59 min, misleadingly truncated to 5 hrs.
Manufacturing Rate Interruption Loss
In addition, as a result of integer truncation (see previous bug, Manufacturing Completion Time Display Bug), every time production stops, you lose an hour of manufacturing. Therefore, for most efficient production, manufacture in large batches (or increase batches while they are still in progress) to keep manufacturing as continuous as possible.
Manufacturing Rate Limit
There is a maximum production rate of one unit of any given type per hour per base. For example, though Alien Alloys / Aqua Plastics require 100 Engineer/Technician Hours per unit, even with 250 Engineers/Technicians you can only produce 1 unit per hour.
HWP Fusion Bomb Ammo Cost Bug
The HWP Fusion Bomb ammunition type, and its TFTD equivalent the ammunition for the SWS PWT/Displacer, appear to have the wrong manufacturing costs. The vehicle version is less portable and less powerful than the personal version (Blaster Bomb), less capable than the craft version (Fusion Bomb) - and costs more than either of the others in total cash and in materials. In particular, it makes no sense that the small missiles on the HWP/SWS use up more of both Elerium/Zrbite and Alien Alloys/Aqua Plastics than the Craft version.
To correct this error, the craft round should have the higher base price, and the material requirements that are currently assigned to the SWS/HWP round. (It's debatable whether the SWS/HWP rounds should be more expensive than the man-carried rounds.) More discussion is on the talk page of this article.