Vossen's Law

The likelihood of a repair or modification working is directly dependent on whether you've reassembled everything without testing it first and inversely proportional to the difficulty of disassembling everything in the first place.

Corollary #1: If it is REALLY difficult to remove and replace, you will break something else just as you get it back together after fixing the first thing.

This really became clear to me working on servers (especially rack-mounted), cars (especially Hondas) and small electronics.  But it seems like it should apply to just about anything that is a pain to get at or disassemble.