Dunfermline Athletic edged a point ahead of Cowdenbeath in their bid to avoid a relegation play-off.
But it took a stoppage-time equaliser to deny them victory and allow Partick Thistle to celebrate their Division One title win with their own fans without defeat.
Thistle had secured promotion last weekend with victory away to Falkirk and the Pars came to spoil their party as they looked to compensate for their 15-point deduction for going into administration.
Geggan put visitors Dunfermline 2-1 ahead at Firhill
All was going to plan for Alan Archibald's home side after 27 minutes when former Dunfermline defender Jordan McMillan raced on to Christopher Erskine's through ball before firing past Michal Hrivnak to put the hosts ahead.
The game was turned on its head by two goals from the Pars in the space of seven minutes.
Ryan Wallace steering Stephen Husband's 65th-minute cross past Scott Fox.
Andy Geggan then tapped in his third goal of the season to put Jim Jefferies' men ahead.
With 10 minutes remaining, Ryan Thomson forced home the visitors' third goal of the afternoon after the Jags had failed to clear Wallace's corner.
Thistle's promotion party looked to be ending in defeat, but Erskine converted Steven Craig's through ball from close range four minutes later.
And the Firhill crowd was treated to a late leveller thanks to Steven Lawless' last-gasp volley five minutes into stoppage time.
It means that Dunfermline need to do no worse than their Fife neighbours next weekend to consign the Blue Brazil to a play-off to avoid dropping to Division Two.
The odds strongly favour Jefferies' side, with Cowdenbeath travelling to face in-form Hamilton Academical as the Pars host relegated Airdrie United.
: Fox, McMillan, Dowie, Balatoni, Sinclair, Elliot (Stephen O'Donnell 59), Forbes (Craig 82), Bannigan, Lawless, McGuigan (Doolan 67), Erskine. Subs Not Used: Daniels, Wilson.
: McMillan 27, Erskine 84, Lawless 90.
: Hrivnak, Millen, Morris, Potter (Wallace 46), Whittle, Byrne (Geggan 62), Husband, Young, Falkingham, Smith, Thomson (Drummond 90). Subs Not Used: Goodfellow, Munro.
: Young, Husband.
: Wallace 65, Geggan 72, Thomson 80.
: William Collum