Today I had to upgrade an old, Joomla 2.5.7 site to latest Joomla, everything went out fine until I had to do the first critical step - the upgrade from Joomla 2.5.28 to Joomla 3.51. Did I mentioned that server has PHP 5.6.25 withouth possibility to upgrade - at least not on short term?
OK, so, wasn't the best setup condition available, but usually this is not a dealbreaker.
And BUMM, I have seen an old "friend", a popup saying:
ERROR: Invalid login