In transferFluid you check that the target tank can receive some fluid but when you do the extraction from the original tank you do not deduct the overflow amount of the previous fill.

Found while looking into CJ-MC-Mods/Overloaded#50

https://github.com/thraaawn/OCXNetDriver/blob/95556d215becae55ccadf9dfaf77cf90d433e884/src/main/java/org/dave/ocxnetdriver/driver/controller/EnvironmentXnetController.java#L184